One of the nice things with the Einstein project is that you can configure how many concurrent GPU tasks to run directly on your preferences page
preferences

"GPU utilization factor of BRP apps" - enter .5 to make your machine run 2 tasks per GPU which is what I found is the best efficiency on 7950, 7850, 660Ti, 670.

Changing that setting does not take place immediately ... not until you download a new task from Einstein.

I usually leave 1 free CPU thread per GPU WU to make sure it is properly fed ... either I set my BOINC preferences to use 1 less thread than I have or I use an app_config file to change the Einstein default of using .5 cpu per GPU to 1.