the sidechains will behave differently depending on whether you are in stereo or dual mono mode. if you are compressing a kick and a bass each in mono, then you will only need to feed the sidechain on the half where the bass goes. if you are running in stereo, you will need to feed the same signal to both sidechains.

another (geekier) option would be to run a cable out of the sidechain of one compressor and into the other, but this will not work unless you unwire one end of the cable and hook it up in reverse. normally sidechains work like inserts - they send on one channel and return on the other over a stereo cable. to get the send from the kick to register as the receive of the bass (instead of showing up on the bass's send) you would have to switch the connector around.