I'm assuming that your HGTV feeds are both east coast.
The SD feed here is west coast and the HD feed is the east coast feed (3hrs ahead). So I never get the 'Watch in HD' choice because it happened on the HD channel three hours ago. It seems that the moon and guide come into alignment tonight at 9PM and both feeds get into sync for a while both by show and episode. Until midnight then they are out of sync again. Have to remember to double check.
Also only about half the programs on HGTV-HD are labeled as being in HD in the guide.
There are a couple other SD-HD channel pairs that work kinda odd but not to any great distress. Most everything here works like it should, ie: the 'Watch in HD' button shows up for the matched pairs.