I don't think a 4 foot tank is big enough for two beardies - personally. I would want at least a 5x2x2 for two female beardies and you still have the possibility that they would fight and you would still need the spare set up in case this happened.
Males are very difficult to keep together. They are very likely to fight. It would be the exception to the rule to find two male beardies housed together that don't fight and just because they don't yet doesn't mean they never will.
Beardies are not sociable creatures and you would need a very big habitat to try it. Not worth it in my opinion.
Since you would need a back up set up in case fighting occurs you'd be as well just getting another set up and keeping them singularly.
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