I've known this exploit for a while now- since the first day of the midsummer event, and i thought it's time to tell you guys about it.
What you do is simple.
1. Get one of the two tabards
2. Delete it.
3. Go to the tabard vendor in Stormwind
Here comes the tricky part - it might take more then one try to figure the right timing.
4. Ask for a new tabard, and BEFORE you actually receive it, talk to her again and ask for the other tabard. If you did it right, you now got both tabards at once!
5. ????
6. Profit!
NOTE: this works for both the midsummer tabards and the illidari tabards!
proof: