Luke goes in to the specifics of what he looked to accomplish in joint practices. Basically they may work with someone on the opposing team to stress something they feel is a weakness in their own game in a different way that can be done with someone they are familiar with. That they can have predetermined stresses but since neither wants to get injured and it has absolutely no bearing on anything there is not the level of concern that would be in a preseason game. He would also even chat with opposing OC.
It seems like an interesting nuance among the vets and savy pros.