I guess it would differ depending on the style that you're brewing, but what I've read (in my quite admittedly limited time brewing!) is that Tettnang, Saaz, and Spalt are at least to some degree interchangeable. I know that many styles depend on the minor differences, but since your recipe calls for both and you can only get one... perhaps you could just try two additions of Saaz?
I'd assume that the recipe calls for them to be added at seperate times anyway, and even the same variety will give different characteristics at different brewtimes.
ETA: I realize that aromatic vs bittering is obviously an issue... but all hops taste pretty bitter to me, so I'm not sure if the matter of degree is a problem. After all, all beer-making knowledge today is a matter of trial and error, and obviously there are at least as many personal preferences as there are styles of beer