Hey John,
For this effect, you will utilize the clean feed as well. Assuming you already have your clean feeds set up for MLE 1 and 2. First build your chroma key effect on key 1. Then in key 2, use auto select key and key your double box background. In key 3 you will build your 2 channel DVEs for your boxes. Select a studio camera for the first box, and for the second box, the clean feed of that MLE.
If your box background is a keyable background, i.e. with two hole cut out and need to cover over the boxes, just reverse the keys n key 2 and 3.
#Vision