Roodee,
You need a third tank mate that will difuse the fighting. Believe me, when ever the third tank mate arrives the fighting can stop.
My six line was a menace in my 12 gallon, would chase nemo here and there and bother him.
I then fell in love with the orchid dottyback from the red sea and brought him home. The sixline went crazy trying to size him up and they were more interested in each other than the clown.
Since both were of equal size they evenly matched each other and no one become the boss.
Today they live in 72 gallon reef and still confront each other, occasionally but back in the 12 gallon, it was the crevices in the live rock that helped break up the fighting.
When the six has someone to contend with, he will lose focus on the other...
This has been my experience in the past,,, on many tanks....
MG