Some even seem to think that Catholics are trying to stake out a middle position in the debate, but the Catholic "A and B are both true" predates the concept of forensic justification, and even predates Pelagianism (although it certainly became fleshed out much more once Pelagianism entered the scene).
Once Saved? Or Always Saved?
When we speak of "getting saved," we're talking about this from a temporal, and thus, a human perspective. As a Book of Life issue, we're either predestined or we aren't. But we're predestined or aren't because of something. God knows the sum of all of our actions, of all of our obedience, of our faith. So even though God knows what we're going to do, we're still responsible for our actions, since He permits us to do them (rather than forces us to do them). This is what's known as the Permissive Will of God. A loving parent lets their children learn from mistakes, and a parent who tries to do everything for their kid (because hey, I'm the parent and I can do it better), can be overbearing and stunt the child's growth. God finds a way to work with us, rather than forcing us to do His will.
But if "once saved" is true, then we can necessarily go (with the help of Christ) from a state of not-saved to saved. And this movement (from not-saved to saved) is something foreseen and predestined by the Book of Life. In other words, there are some people who are in the Book of Life who are not yet saved, and some who are not in the Book who are not yet damned (and who are currently "saved," in the sense that they believe and are following God, but will someday cease to do so based upon their own fault).
No Longer Slaves, I Call You "Friends"
As far as I can tell, the key to understanding A and B both being true is this: sin is slavery, while redemption (or "ransom") brings about freedom. This is very much the way the Biblical writers view it: Romans 8:15 tells us that in becoming saved, "you did not receive a spirit of slavery to fall back into fear, but you received a spirit of adoption, through which we cry, 'Abba, Father!'" He contrasts this redemption with the slavery of sin elsewhere as well (Galatians 2:4, Galatians 4:23-25). This freedom from slavery is at the very crux of the atoning sacrifice. "For even the Son of Man did not come to be served, but to serve, and to give his life as a ransom for many" (Mark 10:45).
Return to Eden
When we're in a state of slavery, we cannot simply choose to not be. But once we're ransomed (Mark 10:45, cf. Isaiah 35:10), once we're "purchased" (cf. Revelation 14:4), once we're "bought at a price" (1 Corinthians 7:23), we have freedom. This is the freedom either to walk in the path which God has laid out for us, or the freedom to return to sin. It is a return, in some sense, to Eden, which is why Christ is "the last Adam" (1 Corinthians 15:45), while Mary is "Woman," or Eve (John 19:26; Genesis 3:15). But being in Eden doesn't mean you can't sin, and even fall from grace (as Eden's original occupants show mightily).
This isn't just theoretically sound. It's also thoroughly Biblical. First, we're called and encouraged to persevere to the end (1 Timothy 4:16; Hebrews 10:36; James 1:12; Jude 1:21): if OSAS is true, there's no need for such instruction. Second, 2 Peter 2 makes it abudantly clear (in my opinion) that OSAS isn't true. 2 Peter 2:1 reads:
But there were also false prophets among the people, just as there will be
false teachers among you. They will secretly introduce destructive heresies,
even denying the sovereign Lord who bought them—bringing swift
destruction on themselves.
These are people who were "bought" by the Lord, that is, paid for by His blood. And yet, they've introduced heresies and wandered off course. 2 Peter 2:4 compares this with the fallen angels, who went from being in Heaven to Hell. If they can go from being saved to not saved, why can't a human being? 2 Peter 2:19 says that "They promise them [recent Christian converts] freedom, while they themselves are slaves of depravity—for a man is a slave to whatever has mastered him." So these individuals have become enslaved again, contrary to Paul's warning in Galatians 5:1. 2 Peter 2:20-22 says that:
If they have escaped the corruption of the world by knowing our Lord and Savior
Jesus Christ and are again entangled in it and overcome, they are worse off at
the end than they were at the beginning. It would have been better for
them not to have known the way of righteousness, than to have known it and then
to turn their backs on the sacred command that was passed on to them. Of
them the proverbs are true: 'A dog returns to its vomit,' and, 'A sow that is
washed goes back to her wallowing in the mud.'"
So it's not possible for a filthy person to become clean except through the waters of Baptism (including Baptism by desire), and entrace into the Lord's family. But it is very possible for a clean person to get dirty again. This post-baptismal sin doesn't require re-baptism, but merely reconciliation with God (cf. John 13:9-10).
Edit: The Christian Triathlon: Swimming, Boating, & Running
In response to DJ AMDG's comment to this post, I thought I should paint a somewhat clearer picture than I have done so far. Consider the metaphor of a drowning person. Christ provides us a lifeboat. Once we're in the lifeboat, we're saved. But until we reach that distant shore, we still have the possibility to fall off of the boat. We're incapable of saving ourselves from the water (we can't just wish a lifeboat into existence), but we're very capable of falling back in, or falling partially back in, or peering over the edge and wondering if maybe we couldn't do things better ourselves if we just left the boat for a while.
Now Christ, who knows all things, knows the precise passenger list for those who will make it to the distant shore, but that doesn't prevent Him from offering the boat to those who won't take it, or who will only take it a little while. But just because Jesus knows person x won't make it to the distant shore, it doesn't mean person x was never in the boat. When we (as non-dieties) say so-and-so is "saved," we often mean that they're "on the boat," so to speak.
The thief on the cross wasn't "saved" when he was running around stealing and causing havoc. Meanwhile, any number of followers of this Rabbi, Jesus Christ, were saved: they were believing in Him, following Him, and just eagerly waiting for the overthrow of the Romans they were just sure was coming. Flash forward to the Passion, and we see some big role reversals. The crowds (not all of them, certainly, but some) have turned on Jesus, while the thief is in the process of just coming to know Him. If we can say that the thief went from a state of being truly unsaved to being saved, then we can also say the crowds went the other direction (even though to God, the thief was saved and the crowds were unsaved the whole time).
Paul uses a different sports metaphor: running a race. In Galatians 5:7, he says, "You were running a good race. Who cut in on you and kept you from obeying the truth?" These people were on the right track, then got off the right track. There's hope for them yet (which Paul expresses in 5:10), but there's also a risk that they'll continue to be misled. In 1 Corinthians 9:24-27, Paul uses the metaphor again, and this time (in 1 Cor 9:27) acknowledges the possibility that even he could be "disqualified." If that doesn't mean going from a state of saved to unsaved, I'm not sure what it does mean.