Some lithium chemistry's cant be ran with agm or lead acid batteries.
The D4S LTO 6.0 can be though, I have a few different people using them with mixed setups using XS Power D3400s as starting batteries and the LTO in the back by the amps with no problems, no isolators, and no other converters like people are rambling about.
Now if it was a chemistry that cant be ran with agms, then you would need some sort of isolator, but then its still stupid because you just delete the front battery and run everything off the lithium and be done with it. Which is why you see people build dumb wooden boxes the size of a battery and call it a battery delete, all you really need though is just a distribution hub under the hood to connect the factory 12v electronics too. Simple buss bars do that just fine.