What you diagramed is fine in theory, however, depending on a host of items, including whether the two daq cards are in the same computer chassis, you could be setting up a time bomb. Industrial installations typically take their 120 vac mains power from 3 phase distribution systems, and if your cards are in computers supplied from a different leg of the 3 phase supply, or even a different distribution transformer (if they are some distance apart, this is entirely possible, but can be the case even for side-by-side outlet strips), you can have some extreme differences in the ground potentials of the chassis. I know they are all supposed to be referenced to earth/building ground, but dependiing on how diligent the installing electricians were, you can get 30 to 50 volts ac from one ground to another! You would be amazed how often this happens.
Given all of the above, and there are many more situations with the power electrics than can cause your situation grief, I would use opto-isolators ont BOTH outputs, and a single output power supply for the load device.
I am not blowing hot air here. About 20 years ago, I blew out almost every piece of test gear on a service bench under the conditions described in my first example. The only things that survived wre the batter-operated testers, not connected to the bench ac power supply.
Be warned.
Good luck,
Dave
