OK, a bit later than I'd hoped but here's my write-up of the process. I'll take some screenshots at the weekend and format it a bit more neatly, but the general gist of things should hopefully be clear from this:
Creating a DVD with multiple audio tracks using DVD Architect Studio
DVD Architect Studio is a great authoring tool, but one of its limitations is that it does not allow multiple audio streams. This guide explains a method for working around this limitation, and is derived from advice and guidance kindly offered by Captain Khajiit.
Export your project's audio streams as valid AC3 files, whether stereo or 5.1 surround sound. Create an m2v file for your video stream using your encoder of choice.
Author your DVD as normal in DVD Architect, and prepare the files to a destination folder called INITIAL.
Create three additional folders, named DEMUX, REMUX and FINAL. Create a new folder inside FINAL called VIDEO_TS.
Open PGCDemux, and select as your Input IFO the VTS_01_0.ifo in the INITIAL folder. Select DEMUX as the output directory.
Select the following:
Mode: by PGC
Options: Create CellTimes.txt
PGC Selection: Titles
Click "Process". This will not take long, as you are not actually demuxing the DVD, just generating the CellTimes.txt file.
When the process finishes, close PGCDemux.
Open MuxMan and slect your m2v file as the video stream source file.
Select your primary AC3 file as the first audio stream source file. Set the language for the audio track, and set the extension as "normal".
Click the "up" arrow on the Stream select field to configure the second audio stream, and select your secondary AC3 file. Set the language and extension. Repeat for any further audio streams you want to add.
Select File > Import Chapter, then navigate to your DEMUX folder and select the CellTimes.txt file created by PGCDemux. You should now see that the bottom left button has changed to match the number of scenes in your project.
Select REMUX as your output folder, and click "Start".
When the process has finished, close MuxMan.
Open VOBBlanker, and select the VIDEO_TS.IFO file in INITIAL as your input file.
Select FINAL\VIDEO_TS as your output folder.
In the TitleSet section, you will see several items listed. Select each one in turn, and click the "Menu" button on the right. A new window will open titled "Menu of <item name>". You should have a single entry listed under "Language Units" in this window.
Select this entry, and the PGCs section below will be populated with all the components of that menu. Select them all and click "Keep" on the right hand side, then click OK.
Having taken care of the menu components, we're ready to replace the main streams. In the TitleSet section, find the entry corresponding to your main feature - you will see any PGCs contained within the title set listed in the "PGCs in selected Titleset" window below, which will also show the length of the PGCs.
Select the PGC for your feature, then click "Replace" from the buttons on the right. A new window will open - browse to your REMUX folder, select the VTS_01_0.IFO file and click "Open".
If you have multiple features for which you are adding multiple audio streams, the best approach is to first open your REMUX folder in VOBBlanker and make a note of which titleset corresponds to which feature or element. Then open your INITIAL folder, follow the steps described to preserve the menu screens, then specify the replacement IFO files for each feature.
When you are ready, click "PROCESS!!" and wait for the process to complete.
This process has been tested and works - as long as your audio and video files are DVD compliant (you'll know if they aren't because MuxMan won't accept them as input files) you should be fine.
The main shortcoming of this process is that, although it allows for multiple audio streams on a single videostream in the DVD, it does not allow you to create an Audio Options menu. I consider this a minor issue; however, if I find a solution I will add it to this process.