Stereo can always be summed to mono, but you lose any stereo imaging. If you pass your stereo signal into anything mono, you lose stereo. The bigger question is what is and isn't stereo in the signal path?
Your typical VCO is usually mono. You can plug a mono signal into a stereo mixer and you'll get your mono audio coming out the left and right outputs (assuming you're not panning it left or right). But that's not true stereo. To be "true" stereo there has to be some difference between what's coming out of the left output and the right output.
Does the filter need to go after the Black DSP 2 or can it go before it?
VCO> Mono Mixer Input #1
VCO Sub >Mono Mixer Input #2
Mono Mixer Output > Filter Input
Filter Output>Black DSP 2 Input
Black DSP 2 Left Output> Stereo Mixer Left Input
Black DSP 2 Right Output> Stereo Mixer Right Input
(all assuming that the Black DSP can take a single channel input and apply stereo effects to it with the original input being panned dead center)
If you have a VCA like the Intelligel Quad VCA, you can trigger two VCAs with just one voltage. You can tap the fourth output as a summing mixer and then into the Wasp filter and then the Wasp into the Black DSP as described above.
If the filter has to go after the Black DSP 2 then you can't get around needing two filters. You'll also need to split any CV so it goes to both filters if you're modulating any part of the filter.