IMO, the best set up is a large primary bank and a small reserve bank. For instance my primary bank consists of 4 6v golf cart batteries wired in series/parallel for a 12 volt bank with 460 Ah and my reserve bank is a group 24 battery. I use my primary for everything and only use the reserve occasionally to test it or if I drained the primary too much (hasn't happened yet, knock on wood). The primary bank is wired to the #1 position and the reserved is wired to the #2. I just leave it on #1 all the time.
The starter is wired to the common so that it can be used in any of the positions except off. But I would probably never use the "both" setting because if I was going to use #2 it would be because the primary bank is depleted. For this same reason I would not wire the starter battery to bank #2 position. You would have to use the both to start with bank #1. You could get a different switch or a series of switches to make a dedicated starter battery but there really is no need to have such a complicated system.
Another thing to consider is the alternator wiring
. I believe that the typical setup is to have that go to the common of the 1/2/both/off switch. This can be bad for several reasons. You only get the alternator
charge to the selected bank. If you change the switch to get the charge to a depleted bank after starting you could damage your alternator depending on the switch you have. Instead I ran my alternator directly to the primary bank and installed an automatic charge relay between the primary bank and the reserve bank.
Here is a really good write up on the 1/2/both/off switch