Dragon002's statement is correct. The $12 RS combiner/spliter is not designed to pass the 2GHz down-converted SAT signal, but their $24 16-2567 diplexer is designed for exactly what you're trying to do; BW on the SAT side is good to 2150MHz whereas the combiner/splitter probably would only pass half or less of that range.
Frankly, that diplexer looks weak to me, especially for the outdoor-mounted end. The gold plating that the $12 RS splitter has would have also helped the diplexer, but there are other considerations. I'm not sure what luck others have had, but in my experience, these things can really add up to hurt you! I'm in a fringe area. The insertion loss of these store-bought diplexers would be a definite weak link in my system. IMHO, if you have the option it would really be better to just run the second RG-6/U (QS preferred) for the OTA antenna. The material cost wouldn't be much greater, and you'd have the option to add a preamp if it turns out you need one. (I don't know of any way to add the preamp's power onto the same cable that's carrying power for the LNBs.) Otherwise, look for a better quality diplexer unless you have really strong local signals. I haven't checked, but I'm sure you can find a better grade readily available. (I trust the installers have sources for, and actually install, better grade hardware!)
PS Dragon002 - I'm a big Steelers fan, too. Just now getting ABC-HD OTA (23.1 WATM Altoona), wish I knew about it in time for the S-B; they went HD just 2 days before...!