[Delphinus]

A few years back I looked into the possibility of creating this kind of flow but using one pump. It was before OM was around. What I thought about using was a MOV (motor operated valve - electric in this case, although you can get pneumatic-activated as well) on a 3 way mixing valve. The trick is having the right ball in the valve so that as one side begins to close, the other begins to open so that the total flow through the valve remains constant at all times.

The reason I didn't pursue it was the cost of the MOV's was too prohibitive for me. I forget exactly where they were landing in price point specifically but I do recall thinking I would have done it if I could have found one for $200ish so I think it was well above that. The pneumatic ones were cheaper but then who wants to have a compressor going all the time and "psshhhhh!! psssssh!!! psssshhhhh!!" going on all the time in the tank room.