I’m looking at get an off-road engineering dual battery set up. In addition to the kit, they offer an add on jumpstart kit for $349. This seems like a great thing to have if you starter battery dies out. However at $350 I’m wondering if this is a very expensive convenience? Couldn’t you just run a set of jump starters between batteries or is going to mess up the system.
Likewise, one of the portable battery starters could work. I like the idea of being able to push a button in your vehicle to combine the batteries and start the car, but it seems like a big price for that convenience. Thoughts?

i think you can make your own version with those 80 or 200 amp solenoid , it just a giant relay, understand how to wire it up. some people in the car audio world use those to separate the batteries. just make sure you fuse both batteries, upgrade the wires on your electrical.

I don't see a need to reinvent the wheel here.... how many times are you planning to have a dead battery where you can't take 2 minutes to hook up a portable jump-start box from amazon for a ~$100 price range...? Did I mention Portable..? As in you're not spending $350+ to have a dedicated jump start solution just for a single rig..

I have a HULKMAN Alpha85 Jump Starter 2000 Amp 20000mAh jump box from amazon (doesn't seem available anymore) but anything in the similar price/spec range will do just fine... it will jump my 100% dead battery with no issue and only consume 6-9% battery.. I recharge it maybe 3 times annually and throw it into any car I may need it.

Keep it simple and cost effective.

This was also an option I was thinking about.