The easiest way to do this is to make a plug so you can separate the batteries later. If you wire a plug so your batteries are in parallel you will double your capacity while leaving your voltage alone.
This is an idea of what I am talking about ...
If you combine the negative terminals, and combine the positives, you are wiring in parallel.
You will be adding a lot of weight however which will negate some of your runtime but you will still see longer runtime.