Thanks Roger. I wasn't clear in my initial post, but you answered my question anyway.
I didn't mean to imply that I would hook up a single battery to both streams. What I meant to say was: If I had 2 batteries, and I hooked up a 6101 to one battery, and a 6201 to another, which would last longer. It sounds like the 6101.
I have 6201 x2 and 6101 x2 in my tank, and I don't want to buy 4 batter backups. I'm thinking of buying just 2, so planning out which of my 4 streams I should hook them up to.
Do you think it's smarter to run my 6101s on the backup? It is for a 180g SPS-dominant tank.