a 20 gal tank will be fine for a baby but should get a larger tank by the time the baby is about 10" or so. A 35 gal tank would last longer but by 6 months of age, you should have the adult tank ready.
For your other question, you won't be able to put them together right away for several reasons. 1: it is best to quarentee any new addition for at least 3 months and 3 clean fecels (one per month). 2: Dragons that are different sizes shouldn't be housed together as the larger can intimidate, fight and even eat the smaller one. 3: The female one would be too young to mate unless she's at least 14 months old. Most feel 18 months or older is best. And finally, 4: even if the dragons are both female, the same size, they can still fight and not get along, causing stress and possibly injury for one or both dragons. Oh and one more point, bearded dragons don't need companionship from others of their kind to be happy.
Breeding bearded dragons has it's own pros and cons to consider as well.
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PHLdyPayne