Diplexers work fine, but watch out -- they don't tell you a detail that finally matters after all these years.
Check out
[Link: dbs-online.com]for info on the Channel Master diplexer. The larger one, the 8002, does not allow the LNB to put junk into the upper cable frequency range. I have not seen this be an issue with straight cable or with UHF, but digital signals on cable or over the air are affected.
Use the big one.
Ernie