Well, I'm pretty new to this. I just bought aTraxxas Slash 4x4 Ultimate 6809L-3 TSM w/2S 5800mAh LiPo & Charger. I'm wanting to get another charger and believe I'll just get another 2s lipo beings the 3s would be a little too much for now. I just don't know what brand and if I should get one with 7.4V or the 11.1 V. I think I'm set on one above 5000mah maybe (7600 mah) since from my understanding, it'll last longer. Also, would I need a cooling fan for BOTH motor and ESC since I'll more than likely use the 2 batteries back to back and cut they're 2s lipo? Thanks guys!

You're in the airplane section.
be careful just changing batteries when it comes to voltage. Voltage is critical too much and you burn things like your ESC and motor.
Now mAh rating is like a gas tank the bigger the rating the longer you can go

Yes, this is the airplane section, but I'll help you since I drive trucks too.

2s is 7.4v. 3s is 11.1v. 2s can take you a long way as you play with the gear ratios, and you'll definitely break fewer parts that way.

Going up in capacity also means going up in weight. I'll suggest you start with calculating the run time you're getting with the 5800 now and then deciding if you really need more. The heavier battery will hurt your handling, your speed and acceleration, and will make wrecks worse. In addition, the extra weight will mean it takes more power to run the truck, so you won't get as big a boost in driving time as the arithmetic says you should. I'd be a much bigger fan of sticking with the 5000-5800 (or even going a little lower) and just buying more of them.

If the truck didn't come with cooling fans for the ESC and motor, it probably doesn't need them. There are guys who run these trucks in off road races on 3s who push them much harder than you will outside bashing with it. If the power system can take that, it can take what you'll be doing.