#5 - Sept. 8, 2010, 2:29 a.m.
As with most faction specific quests and achievements they
should switch to their opposite faction equivalent upon faction change.
That would be the normal intended functionality. I am not entirely certain if that is how it currently works though so I requested more specific information which I should get tomorrow.
I will bookmark this thread and once I receive my response i will pass it along. I would avoid Faction Changing for the express purpose of acquiring both achievements, even if you are able to do so currently it may not remain that way.