Is it possible to route two mono synths trough it? I think it’s possible with the two bigger sisters.
If what you’re trying to do is process each synth individually, at the same time, then the answer is no. There is no Eventide pedal that will allow you to do this. Blackhole, or any Eventide pedals for that matter, have stereo inputs, but they don’t function as dual mono signal chains, except in extreme cases with one or two algorithms.
It is recommended that if you wish to process two mono synths at the same time with one Blackhole pedal that you use an external mixer, routing both synths to it, and connecting Blackhole to one of the mixer’s FX sends.