I have used both one piece and two piece drysuits made by Kokatat. In order for a two piece suit to be effective, both the top and the bottom (usually a “bib”) need to have tunnels. The tunnel on the top is on the inside of the suit and that on the bib is on the outside. Both of the tunnels have to be carefully “mated” by folding them together in about 1 inch wide folds at least three times.
If this is done, the two-piece suits can be quite effective. If you spend a lot of time immersed in the water, a little bit of water might sneak in around your waist, but it won’t be enough to matter. Using a conventional dry top along with a pair of dry pants that has only a neoprene waist band will not keep you dry. This combo will protect you pretty well from splash, but if your waist is submerged, lots of water will get in.
What are the advantages and disadvantages to a two piece dry suit? On the pro side, the two piece allows the dry top to be used alone. This can be very nice for kayaking on a SINK using a spray skirt for conditions in which you are pretty sure you are not going to exit the boat, or in conditions in which a swim would not be likely to result in significant hypothermia. With the dry top tunnel inside the spray skirt tunnel, and the elastic dry top waist band cinched around the outside of the spray skirt tunnel, relatively little water will get in, even if you have to roll a number of times. I find wearing just the dry top to be much more comfortable than a full dry suit in warm air, cold water conditions. I prefer just putting on the bib to drive to the put-in and then putting on the top before putting on. Of course, you can wear a full dry suit over just the lower half of your body with your arms, upper trunk, and head out, put then the top half of the suit dangles behind you and the neck gasket and zipper can be more prone to damage. The two piece suit does allow more flexibility in that it affords you the option to use the bib with a short sleeve dry or semi-dry top in warmer weather. Finally, the two-piece eliminates the need for the zipper, which can be stiff enough that some people find objectionable.
Disadvantages to the two piece? Well, you need to buy two pieces, for one thing. The two piece suit is a bit more fiddly to get into, and you might need someone to help you mate the tunnels together smoothly behind your back. There might be a bit more leakage if you take a prolonged swim. Finally, the folded-together tunnels does create a somewhat thick band around your mid section. This has never bothered me but I have heard some folks complain about it.