Very interested in this because I am planning on doing the same thing when I get my 125 gal
tank set up. What I was planning on was a 1" line coming off my return pump split off into 2 3/4" lines for my return. I was thinking I would need a larger pump than normal, because essentially your adding more head length that the water needs to travel to return to the tank.
The pump you have now will get the job done (that being just returning the water to tank) but because your splitting the line, your GPH
will decrease. So your 2 return lines may be at 250 GPH
each do to splitting the line and the added head length.
Don't quote me on this, this is just what I have come up with in theory. Maybe someone else will chime in and help out here.