That is what I always used to use, another word is "continuous duty solenoid." Looks something like a Ford/AMC fender mount solenoid, but designed to be "on" continuous. Another thing you could do if you are just manually charging it, is just use a disconnect switch to jumper them together.
The really best way is to use two chargers. That way neither battery will try and pull the other and the chargers can properly sense voltage Whichever battery is your starter battery will undoubtedly be "down" in relation to the other through a race weekend
