The way I achieved this on my vision four started the same way you did...
-MLE 1 and 2 had the same recall making a functional 4 box
-I tied a custom control to the recall, which will punch a specific recall on MLE 3 giving me my last 2 channels
Using this method I always had my 2nd MLE free to take my first live source then had an auto run custom control which would restore MLE 3 to my aux route it is defaulted to prior to the 6 box
#Vision