Yeah... Ive heard of people trying to do this. Simply, it's not worth it, sorry
It's better to just go buy a 20 when it's on sale or something, cause all the work and there's a HUGE possibility that the glass would shatter then you'd have 2 unusable 10 gallons and no place for the little fishes D:
So, it'd just be better to buy a 20 gallon! A lot less work!