The diagram below is one I favor when owners insist on a dedicated starting battery but also want to retain the flexibility of the 1/BOTH/2/OFF.
With this simple but very redundant system you retain all the isolation & redundancy of the 1/BOTH/2/OFF switch yet add a dedicated direct wired starting battery. Simply flip to #1 and ON and your ready to go. When you're done flip both switches to OFF. Label them carefully, Blue Sea sells the perfect labels...
Normal Operation:
#1 = ON (powers house loads)
ON/OFF = ON (powers engine only)
Start Bank Failure:
BOTH = ON (everything is now powered by the house bank)
ON/OFF = OFF (start battery is 100% isolated)
House Bank Failure:
#2 = ON (house bank isolated start bank powering everything)
ON/OFF = ON (start bank powers all)
If the start battery were to fail flip the ON/OFF to OFF and the 1/BOTH/2/OFF to BOTH and now the house bank is starting and providing house loads. Conversely you can still use the start bank to power house loads in an emergency with the 1/BOTH/2/OFF in position #2 and the start switch to ON.
This system is more complicated but ultimately considerably more flexible than some other systems where you must use the "combine" feature which is never a good choice when you have unknown bank issues..