Go with the 20 gallon, the ten gallon will not allow it to get away from the 110 degree basking spot they need. Generally you should not house them together, unless you know that they will not have dominance issues, they are the same size, and guaranteed females.
Just a few starter tips that are easily missed when starting with beardies that id like to cover before you get everything.
No sand, or particle substrates like crushed walnut shells, or calci sand. Pretty much anything from a petstore that comes in more than three pieces.
You have to have a reliable UVB fluorescent tube, or MVB bulb. Do not get a compact bulb, the twirly energy saving looking ones. Id recommend the reptisun 10.0, or a t-mvb. The MVB will last you a year, and the 10.0 will only last 6 months.
Do not forget they eat around 50-100 dusted crickets a DAY, so affording one can become a little trouble some at times. The crickets should be no larger than the width between their eyes.
No meal worms, or pellet diets will work.
- Matt!