Combine SD DVR with HD DVR

This is a really basic question. We currently have a SD Tivo unit who's signal is distributed via an attic splitter to 3 TVs. We want to replace one of the TVs with an HD and get an HD DVR from DirecTV. Can we wire both DVRs to this attic system and have it be available to any of the TVs? What equipment and wiring would we need?
There is so much great information on this website. I just wish I knew enough to understand and use it all.
Thanks lots!