Increase a battery Ah rating
1 - You can't.

2 - Yes; if you put two batteries in parallel, the combination will have twice the A-hr capacity of a single battery. (Actually, it will be a little bit more, since each battery will only have to provide 1/2 the current as it does when by itself, and this is a good thing.) HOWEVER. When you first connect the two batteries in parallel, you MUST put a resistor between them and let them equalize. Then short the two batteries and then remove the resistor. When you start, one will inevitably have a higher output voltage than the other, and the weaker battery will look like a short circuit to the stronger. With high-current capacity batteries, this can provide quite a spark if you just short them.