You're conflating amp-hours (charge quantity) with amps (current). At the same voltage, your controller will draw the same current unless the battery voltage sags enough under load to limit it. So you can have 7Ah or 7,000Ah, but the controller will put out the same peak power at 24V.
Also, adding batteries can increase voltage or amp-hours, but not both. Adding two 12V x 15Ah batteries in parallel to a 24V pack that already contains two of the same results in a 24V x 30Ah pack, not 60Ah.
Adding more lead to any bike is a really terrible idea, though. Save your money for lithium.
16 of these (8s2p) contain more usable energy than the two big ole 15Ah lead bricks, and the listing is for 5 times that many:
http://m.ebay.com/itm/351864366128.