You could do it that way or remove the tiles that are in the doorway and install a marble saddle, 4"x36"

Our guy did a neat thing. WE had some extra tile from the bathroom and he took some and cut diamonds from that and from the new tile and make a mixed pattern. Really looks nice.

Are the two tiles flush to each other? If so, all that should be needed is a grout joint, or you could do as mboston suggested:

If not, your best bet would be Stan's suggestion-- using a marble threshold:

I like the marble threshold, and the exiting bath tile is white, so I do think that will work. The level is just slightly offset after a closer look.

Thanks for the suggestions!

Found it for you. Will post on decorating forum.

Hope this now goes to the top for you.