I built this device some time ago to help me butt-solder two pieces together.
The two little vises are as identical as I could make them. The bodies and jaws were machined as single pieces and then cut apart to make the two units. The vises slide along a 3/8" square rod and can be locked in any position. The cylindrical post fits my Panavise base. It's removable so the square rod can be clamped in the workbench vise.
As shown, this little jig has been a remarkably useful tool. Soldering, gluing and sawing are only a few of the things it's been used for. But it really came into its own when I added a second chunk of square rod perpendicular to the original
using a half-lap joint to ensure that the two vises remained aligned. Accurate orthogonal solder joints are a pleasure now.
I made the vises from aluminum which, in retrospect, was not the best choice for soldering since it wicks the heat away from the joint. Steel would have been a better choice but my concern was to not mar the part surfaces.
I've considered adding a third rod perpendicular to the existing two to allow for three dimensional assemblies but haven't gotten around to doing that yet.