Start with a dual filter, then. Something like the Intellijel Morgasmatron, which can work as a stereo VCF but which ALSO can work as two separate VCFs. You can also use this as a "mixer" to go from two sources to a post-filtered "sum" output. This is what I would suggest, as these filters have plenty of different modes as well as separate modulation paths...which, again, you can apply to BOTH filters if desired.
-- Lugia
Thanks again for the advice, Lugia.
This sometimes feels like a hardware version of Magic: The Gathering. The longer you play, the more different cards/modules you know about. And there are a lot of them. Surprisingly the prices are about the same as well if you want the good stuff.