I would like to add additional flight time to my Slow Stick I have recently built.

Right now I have one Thunder Power 2 Cell
1320 mAh 7.4 Volt pack. You can see it at:

http://www.hobby-lobby.com/thunderpower.htm

If I wanted to double my flight time, do you know if I could simply buy another one of these, combine
the two red wires and two black wires and be ready to go?

I have a Uberall Nippy brushless 1812/100 with a Jeti 18 amp brushless controller.

If anyone can help, it is appreciated...

That will increase your battery capacity. Of course you will also have an increase in weight, so your flight time will not be exactly double. You can buy special connectors made to do just what you are talking about without having to cut into your cables.

I emailed hobby-lobby.com and they said something about a connector to do this, but when I asked if they carried it, they never replied. Do you happen
to know what connector I should use? Thanks!

Hobby Lobby carries the Kokam series and parallel modules.

However, I've found them to be a bit inconvenient. They're awfully large for what they do, and you end up with exposed wire ends that any decent-sized piece of metal can short out.

I prefer to simply buy my favorite connectors and some wire, and make up my own series or parallel connectors.