An Isolator will just let one battery die if you leave your stereo going and the car off, and yes two yellows owuld be better, BUT for the $ theres better batteries out there.
but i have one yellow right now, can i just buy one more? i heard of problems with them not bein bought at the same time? i have a 5F stinger cap, can i use it just so i can see voltage with 2 battery's? will 2g run be enough for battery in the trunk?
If you have a yellow now, is it really old or like a month or two? also what kind of power are you running, also IMO I wouldn't bother with the cap 2 gauge would probably be enough linking batts.