It can be done considering the networks that are there. Just because its in 720p doesnt mean you have to give it a bunch of bandwidth. Plus whats the big whoop? There are stations out there with 3 HD and multiple SD now with channel sharing. Heck KTEN in Ada, OK (Sherman, TX market) runs 3 HD (NBC, CW, ABC). WEEK in Peoria, IL runs 3 HD (BC, ABC, CW). WGEM in Quincy, Il runs FOUR HD stations. And then there is the channel sharing in Allentown, PA where you have three HD and 7 SD stations on one RF station.
If you have a good encoder the picture can look good. Conversely I have a dual HD station (KEYC Mankato) that has CBS -1 FOX -2 and there are times FOX looks like crap due to not enough bandwidth given to it.