I really want my tongue pierced, not only once, but twice. I want to get it done the same time though. Will it make my tongue swell twice as much? Or will it just be the same as getting it pierced once?

I would get both done at the same time, the swelling will be a little more because your tongue will swell over a slightly larger area.

It will swell just a little more don't worry. And it will hurt a little more because of the swelling and throbbing. :)

I just got mine done like almost two weeks ago, (horizontal, one on either side) and it's the first tongue piercing I've ever had; however, being around many, many people who have had one tongue piercing, I will tell you: As stated before, the swelling will be more than a single, but not much more, (certainly not twice as swollen). More assuredly it will be swollen for ALMOST twice as long, (Though perhaps this is because mine were replaced with smaller barbells before they were ready).. Anyway, You'll be fine; from what I've seen and can tell, Its almost the same as one tongue piercing with maybe a little more swelling and pain, for a little longer :)
The MOST important thing to remember is to use Biotene mouth-rinse!! A little after every meal, its expensive, (10- 15 dollars a bottle), Especially if you smoke! Rinse a little after every smoke, you'll thank me later.
Things to remember:
Don't play with it!

Brush your teeth! :-/

Try to talk as normal as possible,( I know its agony, trust me) but not only is it going to help the healing process.

Eating, (or lack there of) is probably going to be your biggest worry. Almost nothing will seem to work. Even stuff like mashed potatoes or bean and cheese burritos will pull and push on your jewelry.. And anything Very HOT or COLD will be agony as well, (which is also why smoking can be painful, in addition to the extra pulling between two barbells) Instead, I recommend a diet of Ramen, (Top Ramen is perfect but MUST be broken into smaller pieces AND you MUST let it cool down to room temperature), you can chew as much, or a little as you want, (I've swallowed many whole packets myself just to get a full stomach), AND Applesause, same thing you can swallow it easy, plus you will need it for nutrition with your ramen diet.
Good luck!
-Curtis