Probably not, or else the blood test wouldn't work. Thoughts?
I can almost guarantee the blood test doesn't work, or rather it only works when they don't care whether it works or not. 1)Didn't work with changeling Martok; 2)if the blood test did work, surely some smart guy would've realized that it's even easier to check and see if everyone's leaving a broad trail of bodily filth like they're supposed to. Seriously, if they lose cohesion over their blood, why not their broken hairs or sloughed skin cells? Or their crap?
"Where are your skin mites, Dr. Bashir?" They must replicate the million squirming little insects that burrow in us, otherwise it would be
trivially easy to detect them.
So, to answer the OP, I have little doubt they could split into two separate humaniform creatures, since they have to split into
trillions of tiny creatures in order to actually achieve their disguise, and the most difficult part would only be forming two brains, and if you can form one, why
not two? Changelings can form two arms, lungs, and kidneys.
The real question, then, is where a Changeling's consciousness resides, because it evidently isn't in their meat puppets. I mean, there's no brain in a plume of fire. Which opens up weird areas of speculation: are their Solid forms philosophical zombies? To be indistinguishable from a human, they'd have to have human information processing gear, which--as we well know--gives rise to consciousness, and either free will or an illusion of free will, but at least some agency. Where are those nerve impulses controlling Changeling Bashir's arms and legs coming from? Not his brain, because the Changeling consciousness is in control.
It's really weird.