Don’t make a dual loopkey. If it’s a 40A battery, that’s well within the limits of an XT90 and 12AWG silicone-insulated finely-stranded tinned copper wire. (Sometimes called “superworm”)
If you can solder then run both 14AWG red wires into a 12AWG red wire and then through a loopkey. Then you can split back into two 14AWG wires if you want to.
Only work on one polarity at a time and tape the end of ALL wires you aren’t CURRENTLY soldering. Remember, you can’t turn off a battery while you are working on it.