Well if it's on one end, I think you will be happy with a single 1". They are cheap and you could change your mind later on if you don't like it. If you are positive you want 2 RFGs I'd run the full flow with the 3/4" and get a smaller pump for your refugium,or run your refugium off the overflow and/or reactor outputs.It’s on one end- it’s an in wall 180. 72x24x24. I have a gyre xf250 on the same end, although I might move that to the other end and put it on the synergy shadow overflow. So gyre on one end and RFG or 2 on the other
https://www.reef2reef.com/threads/neil’s-in-wall-180-with-basement-fishroom.390436/
Post 96 for return, a little later for overflow