If you are only going to keep two batteries, and you don't want the electronics to reset, you have a few choices, like four.
1. You can buy a device which rides through these voltage drops on start, I think Newmark and others make one, has internal power supply. Much re-wiring to do though.
2. Get a third battery.
3. Dedicate one of existing two batteries for starting only, but I don't recommend this.
4. Add a third 1/2/ both switch for your ships power feed. Then before you start 1st engine, switch ship feed over to other battery, then when starting second battery, switch feed back to battery of running motor.
Besides no resetting of electronics benefit, you will get exercise going back and forth to stern when starting twins.
Me, I need the exercise but too lazy, so I take the resetting and beeping, it's no big deal, comes back up all by itself right away. As a mater of fact, my Raymarine black box FF is so prickly, it resets even when I hit the SW washdown pump.
My Garmin setup is rock solid, nothing shuts it off but my finger. You know where my money is going on electronics upgrade next time around !
I'm also too lazy today to figure out the connections you came up with, besides, it can't solve electronics issue unless both batteries are connected at once to the ships feed uponn start, which means you are parralling batts, or the switches are on auto pilot and switch themselves similar to No. 4 above.
Note - the DIY advise above works for state of New Jersey, it works the same for all states in USA, PR, as well as remainder of the world !