And the height measurement was to the subfloor?
I do think you're probably better off to purchase two separate pumps for this. While you can do one pump it will have to be grossly oversized and it creates one point of failure for both your systems. JMO
And to answer your original question, You would calculate the flow needed at the outlet of your 75 and the associated height. Then calculate the flow you want for your larger tank at it's head height then add those together. You also need to consider frictional head loss, moreso on your return line for the 75 but it will be a big factor especially if you're using a smaller return line...