Doesn't the "point of aim" for the 2 taking lenses have to be the same? Would you need a mount that allowed adjustment? Or camera bodies with a high degree of specimen-to-specimen uniformity to allow a fixed mount?
There are (or were) Y style cable releases. But years ago for a project I took 2 identical releases, mounted them on either side of a small piece of wood with tape. I carved out a small indentation on both sides of the wood to hold the collar of the cable release in place. I was able to actuate both releases at once.