Very wonderful question that I have not heard anyone answer! It CAN be done, depending on how much control you want. Sometimes, the dildo will flop around more, but it certainly is a different feel because the testicles will sit in front of the harness, therefore touch the person being penetrated with the dildo.
I recommend trying it, if you have the dildo and harness, and following up with it! I may try doing this myself actually at some point in the future, just to see if it works.