I use RadeonPro to do it. Supposedly you can do it manually too but I don't have knowhow myself. If you need instructions on how to set it up I'll try to do step by step. Btw, if you run into other older games that have strange behavior you may also try to disable the second gpu and see if it helps. I know that Space Empires V has white screen flicker if you don't turn off second gpu. Probably more games that have it, just didn't run into many.
Interestingly enough, DOSBox games seem to work fine with both gpu-s.