The amps I've seen have a "pre-out" in case you wanted to add another external amp. I'm not sure about "jumping". I believe it's a regular "out" that you can control volume with remote and use internal amp at same time.
Effects of using y-cable from pre outs
I have built in single channel sub cable. Older receiver only had 1 single sub output. I'm not about to tear into walls or add more cables. If I upgrade to an integrated amp with stereo pre outs...what effect will using a y cable to connect stereo outs have? Phasing effects? High noise?