A 75 gallon tank would probably be the smallest you could get away with, but even an 80 gallon tank would be better. Remember that when you upgrade to a larger tank that it takes more energy to heat the warm side, so one simple basking light won't be as effective in creating a basking area in a 75 gallon tank as it would in a 40 gallon tank. Two females could potentially live happily together though. There will likely be one or two territory spats but overall they should get along fine - and as a bonus you get to see beardie behaviours that you wouldn't get to see with just one, like head bobbing, arm waving and piling on top of each other to establish dominance.
It depends on your beardies, and your viv. As long as you provide two basking spots, two hiding spots, and have lots of roaming room, you should be fine. I have had 2 female beardies together for years, and they got along very well. I would reccomend however, that you either feed them individually, or supervise them while eating just to make sure each one gets enough food. It seems as though one is going to be more dominate over the other, and will snatch up all the food, and take the best basking spot. As long as they don't have to compete, there shouldn't be a lot of problems.