First off, that is the best looking Rat Rod I think I've ever seen. You should toss more pictures up!
Second, if your current module puts out 3.5A, and it's PWM, I'm guessing it's a constant voltage, and the 0-10.5 if the reference going to the 3.5A PWM Driver. If that's the case, you can do a couple different things.
1) Take a 7A driver that can take a that signal in and pump out a similar 7A signal and run it in parallel to the two turbos. The issue here would be if frequencies didn't match up well and you got weird resonance that made the turbo's not actuate sometimes.
2) You could take the input (0-10.5A) and plug it right into a 7A PWM Driver. You should be able to find this on amazon for pretty cheap. (Or two 3.5A PWM Drivers because that might be even cheaper).
3) You could use an Arduino (or similar) to control a PWM Driver. There's a ton of free code out there for this because VW TDI guys have been doing VGT upgrades for years.
I can't remember, but these VGT's are spring returned right? Or does current need to switch directions? If it does need to switch directions, make sure you get an H-Bridge driver.
