T-Mobile already does something similar in the US with their MetroPCS network. They've started to open the old MetroPCS network up to T-Mobile users and vice versa by broadcasting multiple MNC (mobile network codes) from the same antennas. If a phone call comes through, it'll hand off to T-Mobile's GSM network or the old MetroPCS CDMA network.

I think Orange and T-Mobile do something similar in the UK with their shared EE network and Verizon with their LTE in Rural America partners in the Great Plains/Maine/etc.