Most one-piece composites are actually fused shaft and blades with excess material at the joint. There are actually very few 'true' OPS but then again, the parameters of what's a 'true' OPS differs from manufacturer to manufacturer.
a 2-piece combo involves a retail shaft and blade put together. There are standard/traditional shafts that fit only standard blades and tapered shafts that fit only tapered blades.
The advantage of a 2-piece is that you could try different blades/curves and that replacing a broken blade is cheaper than you would with a full OPS.
Performance differences are marginal and depend mostly on the player choosing the gear, not the gear only
To me, the biggest advantage of two piece over one piece is the cost. After I went through like 3 OPS in a few months, at the cost of $200+ each, I went back to a two piece. Now when I crack or break a blade (I use an SE16 tapered shaft and now the new EQ50 blade), I'm only out the $60+ for the blade instead of $200+ for a whole new setup. And as far as balance and weight, I don't notice any difference...
Performance-wise I don't notice a difference between a 2-piece and a fused 1-piece. If I break the blade on a fused 1-piece I'll generally cut at the fuse point and chisel out the old tenon to turn it into a tapered shaft, if I replace it with a high end blade the performance doesn't drop at all, in fact sometimes it improves if I put a better blade in there
Harder to say with 2-piece vs. some "true 1-piece" sticks. People have different definitions of what a true 1-piece is, but IMO if there's no tenon/thickening of the shaft walls at any point, that's close enough to a true 1-piece (others say its only a true 1-piece if the carbon strands run the whole length of the stick, but I'm pretty sure Busch is the only company to make a stick like that). Sticks like the Warrior Widow and Easton S19 are pretty unique sticks, you won't find a 2-piece equivalent, if you want something like that then a 1-piece is the only way to go (there's a 2-piece S19, but it's totally different than the 1-piece, it's basically using the same idea as the old z-bubbles). But if you're trying to decide between, say, a 1-piece or a 2-piece Warrior Dolomite, then there's essentially no performance difference IMO.
Personally I love tapered 2-piece setups, not so sh1tty/expensive when you break the blade or shaft, and it's nice to be able to switch out curves. The only downside for me is that they're becoming pretty rare, to get a really good selection of blades and shafts you need to find a great LHS (in Vancouver, for example, The Hockey Shop/Source for Sports in Surrey is the only shop I've found that has a really wide selection).