Redundancy. My choices were:Just wondering, why do you want to combine and split the return pipes the way your drawing shows?
1) Two pumps, one for each tank
2) One larger pump that splits to both tanks.
My thinking was if I stuck with two pumps if one were to go out, i can close the valve on the failed one and let one pump do the work (obviously with a much lower flow rate, but better than no flow rate). This buy's me time to repair or replace the broken pump without the tank going fallow for days at a time. I live in Reno, NV and there is no place locally to acquire this kind of equipment, so it would need to be ordered and shipped.
The other option is simply to buy two pumps, and keep one on hand, but that will cost even more because I will need to buy two large pumps that can individually handle the entire load.
Another modification that I am still thinking through would be to have each split of the Y (that goes to each pump) have a check valve in it. This way if one pump fails, or becomes weak, gravity and pressure would automatically close the valve on the failed pump allowing an automatic fail safe. The problem I have with that is check valves are somewhat unreliable, so I'm trying to think of worst case what would happen with that setup. I would probably put a manual valve behind the check valves anyway so if a pump did fail I could shut it down totally.
Basically, if I go on vacation for two weeks, I don't want to spend the whole time worried a pump failed and my tanks have gone fallow. So I'm trying to make sure the critical systems are as fail safe as financially feasible.