It depends a little bit on where everything is physically located and exactly how the main breaker panel is constructed, but my first suggestion would be to put both MTUS in the main breaker panel with their current clips on the appropriate outgoing wires, and power them and a new socket for the gateway from an additional pair of breakers in there.
If you can't put another set of breakers in the main panel, or the gateway would then be a long way from where you want it to be in terms of Wi-Fi access, I would still locate the Guest House MTU there, powered from the Main House breaker and with its clips round the outgoing wires to the Guest House, and hope (!) that the wire-transmitted signals reach the gateway (they probably will, provided that there is not too much noise from the Guest House). Both I and the previous poster say to power the Guest House MTU from the Main House breaker rather than the Guest House one, because there should be little enough LF impedance across the two breakers to make the AC voltages substantially identical in the two places, but breakers do seem to block some HF noise (and signal).
