First, how old are the dragons? How big is the 'large' tanks? What I consider large may not be what you consider large and vise versa. Actual dimensions is best (gallons are rather arbitrary as it is a measure of volume more than actual 'footprint'. A 40 gallon tank can be all kinds of shapes and sizes and still hold 40 gallons of liquid)
Next, I always recommend not housing dragons together.
However two females can co-exist without problems but you do have to be prepared to house them all separately as not all dragons will get along. Also the two females should be the same size or very close (within an inch). Care must be taken to ensure one doesn't over dominate the other, preventing it from thriving. Thus a large cage with plenty of basking space and areas so both dragons can be in idea basking temps at the same time. Extra hides and maybe even separate feeding dishes also prevent competition.
Introducing the two females is best done on neutral territory (floor of the living room is good, or bathroom...supervised of course.) You could also put them into a clean tank (wash and disinfect first, air dry than introduce both females at the same time to the tank.) Speaking of disinfecting...If you just bought this other female or plan on buying one, you have to keep her separate from your other dragons for at least a month, plus a clean bill of health from your vet, including a clean fecal. This will ensure it isn't carrying any disease or parasite that can infect your healthy dragon(s). I recommend three months...as some diseases don't show up right away.
Once introduced you should watch the dragons for some time, to make sure there are no sudden signs of aggression. As both are in a new territory, they may end up just ignoring each other till they both explore the new home, then they may start dealing with each other. As it can take up to a week or more for a dragon to accommodate to a new home, they may seem to get along fine for that time, then suddenly start fighting. If they fight it may be necessary to separate them permanently to avoid any getting seriously injured (ie having tails, toes, even limbs bitten off)
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PHLdyPayne