More than one way to go.
I know you are looking for a single switch to handle 2 battery relays.
I cannot remember ever seeing one of those.
Here is what a lot of builders do.
These switches are available everywhere. In my case, I used the one that came with Van's wiring kit and added one to it. The alternator field function is connected between the 2 battery switches so each switch is a fully functional master switch.
While I only have one buss, this set up allows for easy seperation of essential
and emergency buss if I ever decide this will be neccesary.