Yes. You need to quarantine the new dragon for 2-3 months. Do not put it in with the other dragon. Get a seperate enclosure and wash hands thoroughly after each handling of one reptile to another. Have a fecal done when you first get it, and another at the end of the quarantine period.
We all want to trust the breeders we buy from, but the fact is anytime you get a new lizard there is the possiblity of spreading parasites and disease through your collection if you don't practice good quarantine habits. It's happened to the best. Play it safe and make sure the new dragon is in perfect health before exposing it to your existing dragon.
When you do put them together, make sure there are two basking spots and two food dishes. You want to avoid as much domination for food/light/etc as you can. Also feed them insects seperately, to avoid accidental nips.
As for size, it's not so much just so they aren't small enough to be seen as a food item. Even a slightly bigger dragon can cause a lot of stress and intimidation. The rule of thumb is that they need to be within 1/2 inch of each other. It's more difficult with young ones, they grow at different paces, so keep a close eye on them and make sure one isn't outgrowing the other.
Finally, make 100% absolutely positive they are both female. Mistakes happen. 
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Amanda
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2.0 bearded dragons (Ocho / Domo-kun)
1.0 pictus gecko (Nicodemus)
0.1 kingsnake (Rio)
0.0.1 tarantula (Calcifer)
1.0 ferret (Playstation)
1.0 cat (Wally)
0.1 dog (Tima)