Stereo your pedals, not another amp.

[Preis]

The rEal seCrEt of “Stereo” Rickenbacker. You can simply swap the jack on ANY guitar too. I don’t believe it was intended for a stereo > 2 PU guitar to run on two separate amps but better used to isolate some pedal effects into separate channels. I use a Mutron III and it does not like anything else connected with it. A compressor does the opposite and will just cancel it out. So….I run a stereo ric; the Bass PU to an SB-6 ground Isolator, to compressor, to distortion, to chorus, to a stereo EQ – Ch A. I run the Treble PU to the Mutron III only then back to EQ – Ch B. I use a Y cord from EQ back to front of amp input. Now I can get good effects on Ch-B ...if I hit harder, while Ch – A stays clean and compressed!! Now my selector switch has 3 unique tones but the mix of both is awesome.

 

[bassbrad]

Welcome to TB @Preis believe what you will but there are a number of intentions for stereo outputs on guitars and basses several of which you utilize in your scenario but routing signal to different amps is also an intent. Look at the stage setups of Bootsy, Stanley Clarke, Chris Squire, John Entwistle, Greg Lake and others# and you wll see many versions of split signal routing.
Using a Y cable to combine signals is not the best solution as it can lead to phasing and other issues, you might consider a Boss LS2 as it will allow you to mix the channels properly and even another option for switching.

#Guitar players do even more complicated set ups check out Jerry Garcia eg.

What Is Rick-O-Sound? Demystifying Rickenbacker's Unique Output Jack - inSync