20 AH might not get you thru a whole day's fishing, but should be good for 5-6 hours. I run a 35 AH AGM in the kayak with the GLS-10 and a 93sv UHD, and it lasts all day. Lowest I've ever drawn it down was 12.1 V.
I have not used 2 batteries before, but it should work. You might have to run a ground wire from the starter battery (-) to the screw on the GLS-10 box.