Usually they're either one piece or THREE piece slate-if you can get a one piece in, IMHO they're usually better, but a 3 piece is MUCH easier to move, generally beeswax or similar is used to reseal the seams. A 3 piece will also probably need new felt after you move it. I've never heard of a 2 piece slate, maybe it's something new? I haven't been in that business for 20 years or so.