X1 DVR Tuner count problem

Apologies in advance if this has been answered elsewhere.

I have a X1 DVR and 4 Non-DVR set top boxes.
During regular use one of the boxes tells me that i must "Shut off another box" to watch Live TV on this one.

After some research, i learned the primary X1 DVR box has only 6 tuners (in this case, 5 Tuners used for what we're watching, and 1 spared for what we record on another channel.)

Is it possible to rent a second DVR box (presumably with it's own set of tuners) and return one of the Non-DVR boxes? and if so, what is the rental fee difference? (if any).