Well, I did it that way some time ago. About a year ago, in fact -- I was interested enough in Homestead to want to beta test it on the PTS so I did a full and lengthy load to my HD. I didn't copy it over though, I think PTS is its own load. I had two desktop shortcuts to point to each of the versions.

However, once you've loaded the PTS, the same iteration of it can be used to launch live as well. This matters, because my SSD version had a total fail some time later, the launcher could no longer detect the game, and a repair could not run because it wanted to have enough SSD space for a full second copy of the game before it would even start. My attempts to get Support's aid went in circles (they clearly were not reading older messages in the very short and polite email chain that explained the issue and the steps taken and could not give me a straight answer to save their lives to questions I had based on the original answer, which apparently they could not bother to read ... yeah still fuming ... ). So in the end I deleted the SSD version and have since then used only the HD one. It may be slower loading, but at least it works.

So yes, you can have a PTS version on your HD in tandem with a base version on your SSD. But the PTS will include a live login (they patch separately, btw) and for all I know running it that way is how the SSD version got corrupted.