I too am new to twin screws in model boats. But I have built several single screws, and have experience with full size twin screw boats and their handling.
Here's my wiring diagram. You can see 2 SLA 6 VDC batteries, 2 ESC's, and 2 motors. There is one each receiver, Action Electronics Motor Mixer, and rudder servo (on the left). There are fuses between each battery and ESC. My boat will also have a ramp (it's a landing craft/ferry), so there is a sail winch servo on the right.
I am using the motor mixer for the first time, to give the close-in handling of a twin screw. It will slow the port motor upon application of left rudder, and reverse the port motor for an end-for-end turn. Same to starboard.
The ESC's are Mtronics 25 Amp, so the fuses are 20 Amp.
There is one ON/OFF switch for the whole boat, and a Power Pole connector for battery charging, both shown on the lower left.
Let us know how your build goes, and how the boat handles.