512 TR fuel rail combine

Thats a symptom something is wrong. Performing a modification to hide a symptom of a failed component isn't how you fix cars.
The specification is they should be within 3 lbs and if they are, no greater matching is required for proper operation.

 

One issue here is that these fuel pressure regulators "send" fuel back to the tank thru spring-loaded check valves, so depending on the flow capacity of the return path, the fuel pressure may not be constant for all engine operating conditions. I.e.:

When the engine is using a lot of fuel (so the amount of fuel return to the tank is low), probably all of the fuel return flow will be going thru the return path of the fuel pressure regulator with the lower pressure = the fuel pressure will be this lower value.

When the engine is not using much fuel (large amount of fuel return to the tank), if the return path in the lower pressure fuel pressure regulator can't flow that much volume without causing a pressure increase = the fuel pressure may be higher.

This situation is unlike where they connect the control pressures of two bank together on the CIS models as in that case the return flow is thru two fixed (at any particular time) orifices (and not spring-loaded check valves) so that does give an "averaging" effect. To connect the two fuel rails together, you would also need to change to a single, larger capacity fuel pressure regulator IMO.

 

According to Bosch throughout the range of pressure regulator operation the fuel pressure regulation can go from 3.2-3.7 bar but should still stay within .2 bar, bank to bank. Also unlike CIS Motronic 2.7 has adaptive fuel strategy and will vary injector duration based on long and short term needs, small variations in fuel pressure and or density being among them.