From your question, I am imagining that you wish to essentially double the output - - much like bridging a stereo power amp, correct? If so, here's a couple of thoughts.
First off, unless the user's manual specifically addresses "daisy-chaining or slaving" the two amp outputs together, don't do it. You risk damaging the output circuitry of both amps - - and destroying your speaker(s) if stray voltages due to amp failure reach them.
Second, if you are desiring to feed the output of one amp into the front end of the other, you will seriously overdrive the front end of the second amp. Even if you use a headphone output to drive the second amp, the signal level will be much higher than that of line-level for which the inputs are intended.
Last, what you can do is to split the signal coming from your bass into two signals (using a preamp with two line level outputs is best) and feed one into each of the amps. Each amp will operate in its own into its own bass cab; the result will be essentially twice the output and there will be no chance of damage to either amp.