I have a set up that is similar to what sailingdog drew, but is less complicated. I have all of my charging sources going to my house bank of 2 group 31 batteries. I have a group 24 starting battery. My house bank batteries are connected via a 1, both, 2 off switch that goes to a positive bus. I have a Blue Sea ACR that connects the positive house bus to the start battery positive bus. I have just added an on/off switch to bypass the ACR (had a learning experience this past weekend that required jumper cables).
This allows me to parallel the house batteries during normal operation and isolate one of them if needed. It also allows me to easily disconnect them while doing electrical work. The ACR will connect the house and start banks to charge the start bank when the house bank is above a certain voltage that I cannot remember. If your start bank is to drawn down to start the motor and your house bank is below that certain voltage the on/off switch will allow you to manually parallel the house and start banks.
The draw back to this system is if the start battery is bad I cannot use the house bank to start the motor without first disconnecting the start battery. This is a rare situation and, in my opinion, not worth adding a second 1, both, 2, off switch to allow for total redundancy.
The ACR/combiner switch package costs around $125.
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Iolite Vaporize