As others have mentioned, stick with the 3/4" as the main and 1" as the emergency. The 3/4" duly open at siphon will handle almost 1,000 gph. If you have a 1,000 gph return pump, it will flow a lot less than that after head loss. Oversize the return plumbing to reduce head loss (e.g. if the return pump has a 3/4" outlet, use a bushel to increase to 1" for a majority of the run then reduce back to 3/4" into the tank). IMO, the main advantage of using the 1" drain as the emergency is that it adds an extra safety net in case the main gets clogged. With a herbie, you will need to have a trickle going down the emergency to keep a steady water level. That "trickle" uses up some of the flow capacity of the emergency drain, so if the main and emergency were the same size (and the main siphon were fully open), you run the risk of flooding if the main were to get fully clogged. This is where the bean animal was invented to have an extra emergency drain to account for this. So in summary, 3/4" main, 1" emergency, oversize the return, all the plumbing does not need to be the same size.