If your piping is to be ASME Code compliant, I think that if you were to make the root pass by simply fusing with a tig torch, you would most likely wind up with too much internal concavity to be code compliant. Using filler metal during your root pass would help prevent the concavity.
Also, if you need to fill out and cap the bevel with only two GTAW passes, your elbows would have to be pretty thin, like sch 10 or maybe 20 for instance.
Even on sch 10 material, I would be hesitant to try to fill and cap the bevel with only one pass after fusing the root because the heat input required to lay down that thick of a pass would probably cause burn through problems.
Of course I may be wrong and you may have a talented enough welder to pull it off. I suggest that you try it out on a couple of disposable elbows first.
Tim