wireless test fixture jig?

Any good ideas how I can inject 2.4 and 5.8 Ghz wireless test signals, one at a time, into two different mimo whip antennas? I am thinking of some sort of emi shield box with two compartments and lots of RF absorber foam in each compartment, that the two individual antennas fit into. Unfortunately, the two antennas have to stay attached to the main box.

I was also thinking about some sort of capacitive sleaves I can slide over the two antennas. The outside of the sleaves can be a metal shield protruding from a common metal plate for good isolation?

Any idea how much antenna to antenna isolation I can get? Maybe 20 dB? I need enoough RF isolation to form good virtual dispersive nulls. Thanks
Rich