The two-piece toilet refers to how the toilet is made and installed. All two-piece toilets come with separate tank and bowl allowing for ease of installation but with a seam the naturally occurs when bolting the two pieces together. Typically, the tank meets right at the bottom of the bowl and people can find this to be difficult for cleaning. Other companies like Icera, have thought of this and now have the tank and bowl meet higher and while still seamed, make for much easier cleaning and a less slightly fusion between the two pieces.
All Icera two-piece toilets have a flush rating of 1000 grams per flush; which is the highest rating afforded to toilets for soil removal. The also come complete with metal trip lever, slow close quick release seat and maybe most importantly, have a higher than typical water level meaning 1/3 of the bowl will hold water. This will eliminate the constant cleaning other brands in the industry don't take into consideration with their low flow toilets.