AR 9110 is not a powerbox guys! It is just a powersafe Rxer with two EC3 battery connections, multiple remote Rxers and a switch.
You do not need the AR 9110's switch to operate the Rxer. You only need it to shut off the Rxer. However, I would not use the switch you selected to replace the OEM's switch. I have had two of these Push Button type switches fail on my AR 9210s. They fail safe. But they do fail.
Secondly, the switch you listed is not rated for your application. There maybe a few switches out there that would suffice. But, the tip off here is the use of the JR connectors and the light gauge wiring. That switch you listed is not rated for the application you describe. I am not sure I have seen a switch with two circuits that meets the necessary current rating.
I would just plug the batteries directly into the Rxer and use the given switch or, ... plug the batteries in directly and not use any switch. Smartfly has a couple setups that accomplishes fail safe switching with multiple batteries. Maybe swap out the OEM's switch with one of Smartfly's Pin & Flag switches?
All this being said, now there is the voltage issue. I solve that by using nothing but High Voltage servos and 2S LiPo. Others like the LiFe option, but they appear to forget that 6.6. volts is greater than the 6 volt max. spec on low voltage servos. It appears to work for most folks. Then again, ... who in our game knows for sure when a jet crashes that the extra .6 of a volt did NOT cause the servo to eventually fail? Rule of thumb from my biz is never operate outside of the design specs. To do so is challenging the safety margin. In industry you learn that fast. In our hobby, apparently not so. But we all are risk takers. Otherwise we would be sitting home watching TV all year long!
