The original fuel supply only draws from a single tank and the feed comes from the lower tank on the side selected, using the manual selector on the crew side of the firewall. The fuel selector allows left or right or off. So you must feed from one side to both engines and manually switch to the other tank when the gauge shows "E" (meaning the upper tank is empty) so as to maintain a reserve supply in the lower tank. The fuel not used by the engine is returned to the tank that is supplying the engine NOT to the other tank! The fuel supply rate is 40 gal/hr and at 25 psi.
Now, that's the stock set up.
With your modified set up it appears that unused pressurized fuel from both engines is being returned to the same upper tank and the balance pipe is of too small a size to flow enough fuel to balance the system. If you are going to feed each engine individually then you must send the fuel return to the same supply tank or increase the size of the balance pipe. As your system is still feeding fuel from the respective lower tanks the upper tanks already have a fuel return line fitting it should be a simple task of running a new line from each fuel return fitting on the cylinder head back to its respective upper tank. The existing balance pipe may function (you didn't mention its size) once you are returning fuel to both upper tanks, I suspect it is too small. The lower tank fuel feed is above the level of the pumps so no issue using the lower tanks, gravity is on your side.
Look on page 238 of TM 9-731B and the complete M4A2 fuel system is explained.
The manual can be found here.
http://www.theshermantank.com/downlo...-like-manuals/