dual battery help, best/easiest way

[cochino12]

I am running dual batteries in my plow truck and its staring to give me some problems. The previous owner installed dual batteries but there is no isolator for them and now I am having some electrical problems. Currently the batteries are just connected together with a 4 gauge wire +/- and the plow and accessories like radio...are run off the secondary battery. Earlier this week while I was on vacation after about 6 hours of plowing the plow stopped lifting and the radio shut off and the truck stayed running but only read 9 volts on the meter, after the truck was parked overnight the battery was dead.

So here are my questions.

1. What is the easiest way to isolate the batteries
2. Do I absolutely have to replace both batteries when I do it? they arent matching currently?

Thank for any help