Mary,

Yes, there are two piece drysuits. (Or, at least there used to be. I haven't looked at what's out there for a while.)

More common, though, is the drytop alone. It is often worn by kayakers to keep the torso warm. (The legs are in the boat.) They are typically worn over the top of a farmer john wetsuit and do provide a measure of versatility. I will often wear a 3mm farmer john. Over that, I'll add pile layers as needed on the torso and then slip on the drytop. The downside is that it's not as completely waterproof as a full drysuit and if you spend a lot of time immersed, water does seep in, albeit very slowly.

As for Kirt's comments above about air in the drysuit, I typically squat down into the fetal position once I've got mine on, then pull open the neck gasket to "burp" the suit, releasing excess air.