A single 1" pipe running full siphon with a 6ft drop can flow over 2800 gph. I don't think you want that much flow.
You can join the pipes together but it doesn't really gain you anything. I have a dual overflow system with bottom drains and I run 2 drains separately into the sump. They are not full siphon however.
For your setup you might want to look at using the 3/4" holes for the drains and the 1" for the returns. A 3/4" drain can run 1600 gph.
The siphon is easy to make just have the drain head turned 180 degrees (facing down) and don't use a vent hole. With a full siphon you need at least an open channel drain to help tune it and it is highly recommended to have an emergency drain in case of a clog. Search for "Bean Animal". That is the general design you want.
You are going to need an expensive and powerful pump to be able to flow 2000+ gph at 6' head. It will use a lot of power too.