All you do is keep the two batteries separated when the vehicle is not charging.
When it is charging, connect the second battery.
I do this with a relay controlled by the alternator's charge light circuit.
People that don't have charge lights, and other suckers that are stupid enough, buy a "smart battery isolator" which is a voltage controlled relay. (IE - voltage is above 12.5V or 12.8V or 13.5V and the batteries are connected; it disconnects below <whatever> voltage (which is lower than the connect voltage); and uses various time delays.)
The practice is to keep your main battery always connected. There is no need to disconnect it. (Only the other battery(s) when not being charged.)
And forget stuff you read about connecting a battery oncethe other is full or nearly charged - that's all colorful bullsh to blow money from suckers.
Search my posts for more details.
Don't forget to have protection (circuit breaker or fuse) and EACH END of the battery interconnect.
PS - you only need an alternator upgrade if it doesn't handle the total load. The extra battery shouldn't change that (each fully charge battery only adds 1-2 Amps - ie, 12-30W).