charge 2 batteries with isolator?

Probably not.... There'll be 3x the voltage drop... (LOL?) ie, 1/2 versus 1+1/2 - but that is no big deal if (1) the 4G is good enough & (2) it is the same overall length of 4G or equivalent. (That started more as a LOL, but it has some trivial seriousness....) 4G sounds small for a starter cable, but cable ratings are for steady or long term use whereas cranking is short.... The tell-tale is if the cable gets too hot (warm is normal) or if the voltage drop is so great that it doesn't crank properly. (Be aware that the battery may drop a lot too - a good 12.7V battery may dip below 11.0V etc.) The only other distance impact is for protection. Not that starters are normally fused, but if (say) the switch is a form of protection, now it is 3x the distance from the battery. But if physically secure - ie, negligible risk of a short etc, then that's ok. Starter motors are usually not fused because their cable etc is considered physically secure/safe (at least with standard lead-posted batteries where the post can be the ultimate but unintentional & undesirable fusing point). It is the "[physical secureness" and its risk of failure that determines what is the acceptable distance from a battery and fuse etc. (I often see quoted max 6" or 18" between battery and fuse without regard to the type/size of battery and layout - as if there is some magical fixed distance that applies. The magical distance is so (there is acceptable risk) that "no short/fault happens between the power source (battery) and the fuse".

Well I did find out that the aircraft cable is actually tinned copper, so that makes me feel better.. Glad I found that out.. I had asked about that earlier since i have access to that cable.. I originally thought it was Steel because it looked like it from the outside..

That makes sense! The best conductivity but tinned for solderability or maybe less termination corrosion. Tell them to start using gold instead - even less corrosion - that's unless you cease having access to it.... Thanks for clarifying.... Steel did seem weird... (Did I suggest fatigue? Hmmm)