Is there any diference in the headers themselves, or is the only difference in the y-pipe?

So, if i bought a set of slp single cat headers, will the slp dual cat y-pipe bolt up to it? Or what if i did the same with MAC headers.

It seems hard to find dual cat headers used, but i see single cat all the time.

 

no the dual cat ypipe won't bolt up to single cat headers. The drivers side header on a single cat is much longer while the dual cat header is very short, so it won't move the factory cat. If you don't care about emissions you could just go with the entire single cat setup.