There have been answers from GMs in both directions (do need at the same time/don't need at the same time) and some people have had problems getting it when they didn't have them at the same time (though there certainly could have been other issues) but in general, the vast majority of the responses/proof I've seen says you don't need them at the same time, including my own tracking of the achievement in-game via script (granted that was pre-Cata, but still).
You should be fine. Look up the script to show the achievement tracking and try it if you want to be more sure.