This is one of the many little inconvient pitfalls in the program since it is only version 1.0.
You also have to remember that it is suppossedly idiot proof and that's were this particular limitation comes into action.
If you want multiple audio streams your going to have to invest some money into Professional DVD Authroing software. So if you want the best straight and simply answer is Sonic Solutions, but be prepared to pay through the donkey's arse for it.
You also have to remember that if it is a professional software package it is not going to be as user friendly as DVD-A.
For simply jobs I like to use DVD-A for one main reason, and that is I can create a menu in less than 2 mins. This kind of user friendliness is incorporated for the mass market which is just starting to open up as DVD writers are becoming more affordable to the average home user.
If it will fit, you could always put the video on the disk twice, each with a different audio, then use a menu to select which one to play. Not as slick as the Hollywood multi audio tracks, but it could work.