If you're talking about, like, making your tama last longer without the battery dying on you, it's not such a good idea. Some people have already reached their 99th generation, no problem, with one battery. So I think just one battery is strong enough to last that long.
And besides, some of the tama ancestors to have double batteries (take the P1 for example), but the batteries are smaller and they cost, like, six bucks for a pack of two! One battery is cheaper, and it lasts a long time. I don't think it would be a really good idea.