That is a tricky question about pressure. The GPH coming out of a pipe is affected by many things. This is to include how high up plumbing has to go, how many elbows are in it, if there is a split in the connection...so on. You want the GPH out of the return to match what your overflow is rated at. You always want to have a return pump rated higher than the overflow so it can be adjusted back so it matches perfectly. The more elbows and splits, the higher the return pump will need to be rated at because of this head loss. This is why your friend mentioned needed two pumps. Sometimes it can be cheaper to plumb two in rather than 1 when things start getting really large. This is something that you will have to look into and decide on your own.
There are GPH calculators out there that might be of assistance to you.