How do you wire 2 subs with integrated amps together?

When you start mixing a hodge podge of speakers and amps in a car the results are rarely (never?) good. However, to do what you want to do the most correct way would require a power distribution block that would split one large power wire coming from your battery into two smaller wires used to power the amps. The size of the wire you need depends on the "actual" power (and resulting current) required by your amplifiers. Generally the smallest dual amp wiring kits you'll find are 4 awg primary splitting to two 8 awg secondaries. However, if you buy the d-block separately I would expect you'd be able to get by with an 8 awg primary and 10 awg to each amp since amp'd subs don't usually have very powerful amps. I would recommend a fused d-block because of the change in wire size. If you really want to cheap out you can just connect a power wire from the first amp/subs's power terminal and run it to the second amp/sub, but you still need to be sure the wire coming from the battery can supply enough current for BOTH amps. For ground just run a wire from the ground point separately to each amp. Those should be pretty short wires going to some bare metal on the chassis. You can use the same ground location. For remote wires it won't hurt to just link from the first to the 2nd. For RCA's, if neither amp has rca outputs (a lot of stand alone amps do) your easiest option would be to get a pair of 1f/2f y-cables. Connecting them should be easy enough to figure out - you will probably need a short set of rca cables to go from one amp to the next.