What is a 1pc toilet vs. 2pc toilet?
The one piece toilet is designed so the tank and bowl are cast as a single vitreous china fixture. Typically, one-piece toilets have a lower profile than two-piece toilets. Kohler’s one-piece toilets offer some of the lowest profiles in the industry. Two piece toilets have a separate toilet tank and bowl secured to each other. They are also referred to as close-coupled toilets. Many Kohler two-piece toilets offer a 3-bolt installation system which provides a sturdy, level connection between the toilet and bowl.
The poster above is right. I would add that separate one-piece toilets are usually better. According to sanitaryreview they are easier to clean, have a more powerful flush system and have standard chair seats, which makes them comfortable to use both for adults and kids.