I'm positive I've seen someone attach two tanks together like you've described, but I can't seem to find the link now

. The bottom glass and trim from one tank was removed, and was then siliconed onto the top trim of the other tank. This wouldn't be safe for water, but worked for a terrarium.
It's probably cheaper/easier to keep an eye out on Craigslist as already mentioned, also garage sales and flea markets. A bonus for you is the tank doesn't need to be watertight (assuming no false bottom or water feature), so keep an eye out for 'as is' tanks. In any case, you can also re-silicone a leaky tank and it's less of a disaster if this goes wrong for a terrarium than an aquarium, so you can be bolder than a fish person.
You could also extend the height of your 10 gallon with a wooden/pvc pipe/other frame and screen mesh instead of another tank.
Or you can turn the tank on it's side if height is your primary concern (no space increase this way). Lots of info on the web on these conversions, as well as some ready to use kits. Though dart frog specific, here's a fine example:
http://www.frogforum.net/vivarium-te...struction.html
From what I've read while researching how to build my own tall tanks, if the plastic trim is holding your tank together, you're in trouble. Silicone on it's own does the job. As I understand it, the trim will be for looks, protection from bumps on the corners/edges, even dispersion of the weight to the edges of the sides of the tank, and a place to rest the lid.