Even without the RCA out, you can get the same effect, by placing a Y at the end of the cable run. One leg goes into the sub, and the other continues on to the next sub. You're just moving the Y from inside the sub to outside.

So yeah, if you have two subs which are nearer to each other, than the receiver, use a Y at the end of the cable run from the receiver. If both are closer to the receiver than each other, place the Y at the receiver end, and run two cables.

Pioneer PDP-5020FD, Marantz SR6011
Axiom M5HP, VP160HP, QS8
Sony PS4, surround backs
-Chris