you would need an isolator for two seperate batteries ... i would recommend doing an alternator before you get an isolator and a second battery though since our alternators have enough trouble supplying for the single battery ...with a mild to big system... in there now...

:edit:

 

Any news on that model number?

 

Model number 75-35-925... Redtop Starting.. 910 Cranking Amps - 720 Cold Cranking Amps, 90 amp reserve.

 

OK..I went and bought a Red Top today the exact same one that SWF 05 tC purchased and was a bit disappointed. First of all it was way shorter than the stock battery and had trouble trying to figure out a way to keep it in there tight, but luckily
i didn't have to keep it in there long. before i left work i checked the voltage on the battery just to make sure it was charged and it was so i connected it in my xB and let it run a bit before i turned on my system and low an behold the lights dimmed
worse than with my factory battery. what to do what to do.......i'm going to take it back in the am luckily i only paid 56.00 at work for it and ill see if there is a taller one with more cranking amps or something.

 

Sounds weird to me... I don't have a single problem with mine and it stays in the tray fine. They have like 4 differetn sizes of redtops, maybe you'll find something that works better for you. Or maybe you just got a bad one.