Carrying two handhelds, as noted above, could work for you. That has the added advantage of protecting your palms if (when, really) you go down hard.

If you're absolutely convinced you want a hydration pack, Nathan also has the Synergy, which has two bladders and a mixer valve that lets you select or mix the two as you drink. You can fill one bladder with water, the other with electrolyte-drink-of-your-choice, and then select one, the other, or a mix of the two. I haven't tried it, but it seems like an interesting idea.

Here are a couple of reviews:

http://munisano.blogspot.com/2009/09/product-review-nathan-synergy-hydration.html

http://runjunkie.blogspot.com/2010/09/rj-review-nathan-synergy-hydration-pack.html

Don

Disclaimer: I have no financial interest, but I have been a product tester for Nathan.

For trail running I would strongly recommend carrying hand helds.  They are the quickest and easiest to use and also will break your fall if or when you take a tumble.  They take a little while to get used to but I prefer them over bottle belts or camelpacks.

Here is a review of the one I use.  REVIEW

You can either just fill them up with gatorade or you can use gels.  You can fit about 2-3 gels in the pocket on the Nathan's handheld.  If you run in Costa Rica I'd be sure to make sure you are drinking a lot of electrolytes.  Another option is carrying a few salt tablets or electrolyt tablets.

I use S-Caps