Do you have a 20g Tall or Long?
The long might fit, but the tall looks like it would hit the bottom of the other tank. That's just judging by the picture with the tanks in it.
I'd just go to the store and measure the stand to see if the tank will fit.
10g Heavily planted - 1 mating pair of Apistogramma Cacatuoides "Orangeflash" and a whole lot of MTS