Well, the rule of thumb is 6 square feet of ground space for one, add 2 for each additional. So I guess it depends more on the dimensions of your cage than the gallons. You want it at least 4ft x 2ft, and bigger is even better.
Keep in mind when housing two female beardies together, that they should be the same size. If you have one now that is small you need to get another one that is the same size, and if one gets much bigger than the other seperate them. Size differences can bring up dominance issues, among other things. Also, even if they're the same size there is the chance they won't get along and may need to be seperated anyways. So just keep that in mind.
Good luck. 
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Amanda
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2.0 bearded dragons (Ocho / Domo-kun)
0.1 kingsnake (Rio)
1.0 ferret (Playstation)
1.0 cat (Wally)
0.1 dog (Tima)