Hi,
I may be reading the posts incorrectly, so maybe repetitive. If so sorry....
The switch at the helm is just for the sending units (could be different on newer boats). You can toggle to see how much fuel is in each tank.
There are directional pointing switches (main and aux) in the aft area to allow you to run each motor off the main or aux. Just chase your fuel lines back and you will find them.
Personally I'd run them both off one tank and when done switch to the other and you have a trigger to refill. When refilling fill both and your done.
I did it this way and never had to worry about what tank I was using. I alternated fuel use each tank so I never had fuel in the tank too long.
A purest will say, that's bad because if you get bad fuel both motors will be affected. Yes, guilty as charged. I guess you could run one off each tank (best if equally sized like the marlin) and when you get low on the smallest tank refill both. I would find that a PIA as you have to fill more often if your boat has 2 different size tanks and you only are filling one tank a time. Too much management for me. I have seatow for such snafus, but never needed them in 20+ years on the water.