There's certainly no issue with connecting two DVRs, or one Genie + one DVR, to one TV. The TV just needs sufficient AV INPUTs for both devices.
The main reason that DirecTV doesn't support multiple Genies on one account has to do with the Genie mini-clients. There's currently no way to assign mini-client boxes to different Genie DVRs. Additionally, the fact that Genie's have five tuners, and require 5 SWM channels, means that you must have at least a SWM16 setup, which most folks don't have.