I would really consider separating them. The trouble is the runner drake weighs a good amount more than the female call, so he could hurt her or drown her.
We had runners and calls, and they are both some of my favourite breeds of ducks, but ours did have separate runs to live in. The only exception was when thanks to the fox we ended up with only 1 female call and 1 female Muscovy, they lived together, and absolutely loved each other. But of course two females were fine. If you only had female runners I would say leave them all in together.
Beth