I asked the same questions and everyone said to use to 2 switches, 2 batteries, 2 receivers, I am currently rigging all of my gear at the moment for a Twin project as well. I would recommend the Life Batteries they hold a lot of juice and seem to work very well on various boat applications. Hope this helps
I have seen people recommended 2Qt 6-Volt batteries, 2Qt. Receivers, 2Qt switches. In the past I have been using the Venom batteries mostly in my gas boats and have recently put a new Venom hump battery in my Mongoose Twin for the steering. Try checking out Tower Hobbies they have different shapes and sizes and shapes in 6-Volts. They have held up for a few years in my boats, i.e. 3 or 4 years so far on some of my gas boats and 7.5 JAE. I keep track of the condition on all my 4.8 & 6.0 batteries by cycling them with my old Accu-cycle charger before each time I hit the lake.
On my Mongoose Twin I did 3 of everything 4.8 for throttle & 3rd channel and then 6.0 for steering. The problem was that I was getting weird interference on the throttles of the motors with my FM radio.