If you got a mated pair it is not only possible, but advisable. A pair will keep each other happy and you will get to see their courting rituals. You should not keep 2 males of the same species in the same tank, unless it is a very big tank where they can each establish their own territory. A 125 is not big enough, IMO. I belive having 2 males of 2 different species is okay as long as they don't look too similar in pattern and coloration, and I believe 2 females of any species is okay. I would wait for someone else to confirm that before trying it though.
Do not keep same sex mandarins, or any other dragonets in any size tank. They will attack and eventually one will be killed. A pair will require a huge amount of natural food and no competition from fish with similar eating requirements.
I have had both types of mandrins in a 150, which is only a little taller than a 125. They both were great together and they were eating spectrum too. :wink: