Supplying fuel from one tank
Gary's explanation sounds like the system my 58 had too.
Remember about the return lines. I don't think my 58 had any selection on the return lines, especially if both tanks are full or nearly full.
If you draw both engines from one tank, but leave each engine returning fuel to it's 'normal' tank, it can over fill that tank and send fuel out the vents. It might be best to experiment with 1/2 full tanks and watch the fuel levels.