If you are mounting two batteries aft because of use of an angle valve engine, then run #2 wire from the common connection on your two master solenoids forward to the starter solenoid.
My first RV-8A (parallel valve IO-360) had two PC-625s mounted just aft of the firewall, one on each side. After upgrading to a nice Hartzell CS prop I found it a little nose heavy when solo.
The new RV-8 project also has a parallel valve engine and Hartzell CS prop. After doing the math I determined that one PC-625 mounted aft and one PC-625 mounted in the forward baggage compartment well would yield a good W&B. I run #4 welding cable from the aft battery master solenoid up to the common output of the forward battery master solenoid. From there I run #2 welding cable to the starter solenoid and then starter.
Carl