i gotcha now, i would not use two ring terminals and just hold them together with a bolt. this will definitely add a choke point and resistance, we do not want this for our connection inbetween batteries and the alternator. another option would be to buy some 0 gauge ofc copper and make a new run, then cut up the old run to make all your new connections and grounds.
that kinda ***** but thats what id do, you could also just add a second run for the 2nd battery but i really dont think you need to have two runs, 1 run of 0 gauge ofc is plenty and less messy with less fuses