I was thinking the opposite. Let the 3/4" be the full siphon, possibly without a valve, and let the bigger pipe have what's left. Seems safer to me. That way if(when) a blockage happens, there is a unrestricted 1"pipe to take over. Compared to something clogging a restricted 1" main and only having a 3/4" emergency pipe. Wouldn't matter with that return pump, but seems more idealI would use the 1" as the main drain and the 3/4" as the emergency drain.
Not sure what the pump is or how much head loss you are looking at but the return line may need a valve on it.