You don't need backing plate, if where you are mounting is ¼ inch or more in thickness, but if my gunwale was only ¼ of an inch, I would still use backing plates. Also, if you have an aluminum gunwale, be very careful, as I have seen them ripped off by downriggers!
My downriggers are wired to my house main bus. By doing this, I control which engine or engines charge the house battery bank, which also consists of my #2 battery. If you don't have a house bus you can run them directly from the battery selector switch, which also gives you the ability to charge either battery and run the downriggers from that charging battery.
Concerning running the battery switch in "both" position. I highly suggest you "Google" the recommended position, and when that should be done, and why you should ran in the #1 or #2 positions, but "never" in the "both" position! One main reason is if one of the batteries does go bad or is drained, the charged battery will transfer its charge to the drained battery! Can you say "dead in the water"? Another, is you could and probably do have miss-matched batteries, which can cause damage to the batteries, or at least reduce the battery's overall life?
I have twin merc 225's. The engines have 45 Ampere Stators. If they are idling @ below 1000 rpm they produce as little as 17 amps (each) of charge to the batteries, out of that 17 amps, one or both are trying to charge the house battery, as both engines are rigged to charge the house battery. #1 engine charges #1 battery, #2 engine charges #2 battery. At 1000 rpms they each produce 33 amps and at 6000 rpm, each produces 41 amps. So, while I am running/cruising and no amps being drawn from the downriggers… No problems! Everything can be running and the batteries will be charging, just fine.
See the Chart below!
Now tied to my house battery, I run all my radios, TVs, and electronics plus:
1. Three electric cannon, Mag20 downriggers, each rated at 25 amps (full load) and this alone can be as much as a 75 amp draw at once, especially if I use the 20-pound weights, they are rated for.
2. Norcold Refridgerator and Norcold Freezer (which is great for keeping bait frozen), with each using approximately 30 amp hours/day and less than 10 amps.
3. Fresh water pump.
4. Fish well pump.
5. Live well/Raw water washdown pump.
I watch what is running closely, while trolling, especially when I am only running one engine! Many times, when idling, I will turn my #1 battery off and run both engines in the #2 position, to provide more charge to the house and #2 battery. If everything is running, the engines do not stand a chance in maintaining, let alone charging the batteries!
If you think, my engines are going to produce enough charge to even maintain the charge in the batteries while trolling and using downriggers… You Are Nuts!!
I highly suggest you find the amount of power YOU are using; versus the amount of charge your engines are producing, or sooner or later you will find yourself, "dead in the water"!