(As the most conservative measure.) I suggest that you keep, at a minimum, enough distance so the hoists have double the proper "crack separation" between the two of them. The manufacturer should give you a figure for a minimum distance from the posts to any cracks in the floor.
For instance, if they give you a crack distance of 8" from anchor to crack; make sure you are 16" from anchor to anchor between your two lifts. This should prevent stresses in the concrete from communicating between the anchors and causing any potential cracking any in your floor. If you can stagger the lifts, (so they aren't side by side) that might be beneficial for you if you want to space them closer together laterally as the anchors may be further apart.
The pull-out stress the anchors impart to the concrete is concentrated in the shape of a cone:
A lot of the specs about anchors and concrete depth for your lift are driven by this effect.
The lift engineers will design the base plate and anchors such that they plan for overlap of these cones:
^ But it's hard to know what would happen with two sets side by side, taking different loads, in different ways, at the same time.
So, getting this conservative "crack separation" should give you a lot of piece of mind that the lifts' stresses won't interact with each other. Or - you could have a rule where you are only lifting with one (at a significant height) at a time.