I agree with those saying keep one yellow tang and return the second. While two may coexist for a while, it is likely that at some point they will fight with the intent of killing the other, and you will be left with one or none.
You shouldn't add another tang after the yellow as they can be very territorial and are likely to kill any other tang that is added later. For that matter, the yellow tang you keep may harass or kill many other fish you add to the tank, so you will have to be careful in selecting fish.
What other fish are in the tank now and what else do you plan on adding?
You should be quarantining your fish before putting them in your display tank. Many use a simple 20 gallon long tank (you can buy it for $20 during Petco's $1 per gallon sales), plus a hang-on (HOB) filter like an Aquaclear, a small heater, and a small powerhead. The fish should be quarantined for at least 4-6 weeks. I and many others treat fish during quarantine with PraziPro and Cupramine whether or not we see signs of disease. Read the threads in the fish disease forum to learn. Too many people don't quarantine, have things go fine for a while, and then a fish brings a disease in and wipes the entire tank out.
I would take one yellow tang back (or sell it to a fish store for future credit) and then not add any more fish for at least 4 weeks. In the meantime, I would set up a QT, soak the HOB filter sponge in your sump or DT to seed it with bacteria, and put your next fish in quarantine. Ask here first for suggestions on fish that your yellow tang and any other fish in the tank might not harass.
