Sweating means losing water and electrolytes--everyone's sweat is different (i.e. differently "salty"). At some point, it may lead to cramping. I've never had this problem, so I'm not really dialed into it.
When you initially mentioned GU, I assumed you were talking gels. But when you mention you mixed it with water in a bottle--are you using the Electrolyte Brew?
Rather than add a third bottle with something else in it--which seems like too much to keep track of--you could try salt tablets, or find a higher sodium/potassium mix product. If you can go to a store like REI and look at their nutrition products (they have a pretty good selection), a local running store, or even Google "electrolyte comparison chart". You'll get some results comparing various products out there.
Yeah, it's a little confusing, but that's why you start now. You'll get dialed in to something that works for you.
Maura
Thanks Maura! And you were correct in assuming gels, I dilute my GU gels in water, I really cant stand to eat them straight from the package.

And I agree, a Third bottle seems a bit intimidating. So if I were to try a salt tablet, would that replace the idea of NUUNs?
- Heather