Get the female you have now eating entirely prepared foods such as mysis shrimp which are more nutritious than brine. Then you could set up a small 20-30g tank with sand and 10-20lbs. of live rock, grab a male mandarin and stick him in the small qt-like tank until he is grown to at least an equal or bigger size than the female. Dose the small tank with pods
and try to train him to eat prepared too. When he is the size that you like, add him to the 120g. You would most likely want to pick one spot in the tank and stick a jar or another object that will hold food without the water blowing the container/food away and get the mandarins trained to go to that one spot for food. A mature 120g is fine for one mandarin on pods
alone, 2 is extremely pushing it. The sixline probably grazes on some pods
too, so having the mandarins primary diet be prepared foods, you would avoid the possible trouble of starvation. Hope this helps. Many other ways to do it, but this is just one that comes to mind.