Shared GND on 2 ATX Power Supplies
ATX power supplies have floating outputs.  If you connect the output common returns (grounds) together, the outputs will remain floating.  They will float together.  Whether you connect the grounds or not, changing the voltage on one unit should not affect the other one.  If you connect two power supplies in parallel, however, you will have load sharing problems.

At the same time, I don't know exactly you are wiring them together, and what modifications have you made to the 2nd unit in order to get 0-24V output.