I don't own Slingmedia, but I do understand that they must be plugged into outlets that are on the same phase of AC current in the household.. Most houses typically have two phases, with roughly half of the circuits (and wall outlets) in the house on one phase and the rest on the second phase. Determining which phase your two Slingmedia are on when plugged in -- other than by trial and error in moving one of them to a different wall outlet -- can be tricky; as it involves removing the cover plate from your circuit panel box and viewing which of the two main bus bars each circuit breaker is connected to and at the same time figuring out which circuit breakers control the specific outlets into which your two Slingmedia are plugged. It is not so simple an answer as "well, the circuit breakers on the left are on one phase and those on the right are on the other phase".
I would advise consulting an electrician -- attempting to figure out the phases yourself can be very dangerous -- electricity can and does kill people when poking around in circuit panels.