Sorry to not answer your post.
I have been a little busy with my mothers funeral.
As Steve said, 1 1/2in. is a good size. That would allow an 1 1/4in. Durso. A 1in. Durso on a tank I have, can handle my Iwaki 20, about 500gph. A pair of 1 1/4 overflows can easily handle a 1000 gph.
A Mag 12, split into two, will not have much current. But it depends on tank size and occupants.