What you can do is install two one-way check valves, one on each tank and both of them plumbed to the water pump inlet. This way, the water pump can get water from either or both tanks, but one tank cannot fill the other. This solves the problem of having water flow from one tank to the other, but does not ensure that the water pump pulls its water from the two tanks equally. A device to do just that is common in hydraulics but I do not know of one for water. Will have a look, tho.
1975 32' Flybridge Sedan, twin Perkins 6-354 diesels, 1:1.53 velvetdrives, 16 X 19 props. Merritt Island, Florida