AFAIK, both only do slave MIDI sync. And the minibrute doesn't have any sync IO exactly like the volca does. So no way to directly link them like you could link two volcas with the analog sync ports, or either the minibrute or the volca to another MIDI sync master device.
But... you could do something interesting with the gate output of the MB. I believe the volcas expect a rising or falling edge (you can customize this) gate signal for every two steps, i.e. an eighth-note pulse if you're using each step of the volca sequencer as a 16th note.
So you could in theory run an arpeggiation on the MB, and get a volca thing syncing to that. You'd just have 2 volca sequencer steps per Minibrute arpeggiator steps.
You could also do the reverse, using the volca sync output to drive the MB gate input, but that would be less interesting, because it would just trigger the last note played repeatedly; the MB can't (at least right now, not sure if it could be added in a firmware update), say, make the arpeggiator run off of an external clock instead of its internal tempo (or the tempo specified by MIDI sync)
The thoughts above don't consider the possibility of modding. The volcas have a super easy MIDI out mod, apparently, with the relevant connection labeled
clearly on the PCB. I imagine it would be ridiculously awesome to route the volca bass's sequencer to the minibrute. And then you could also run the volca bass thru the MB filter...
I plan on getting a volca bass sometime.