So there is a open source sound toolkit, SoX Sound eXchange, that is a command line tool that can do all sorts of audio processing. You can download it from
http://sox.sourceforge.net/Using it you can convert files from mono to stereo pretty easily
sox "left.wav" "right.wav" -M "stereo.wav" remix 1 2
Replacing the file names with your wav files. You can even do things like take a stereo file and merge it to mono and then make it the right or left channel.
All nice, but it has no interface. But that's where Windows "Send To" right click menu comes in. In the Windows Explorer when you right click on a file (or more than one file) there is a menu item with "Send To" that allows you to send the selected files to a program as arguments. And it's really easy to add your own command to the menu. So all I needed was a batch file that would take two file names and do the processing using SoX. So I wanted it to handle any kind of input (mono or stereo) then do the merge on stereo and create a stereo output. So here's the DOS batch script I came up with:
@SETLOCAL ENABLEDELAYEDEXPANSION
@for /f %%i in ('sox --i -c %1') do set file1=%%i
@for /f %%i in ('sox --i -c %2') do set file2=%%i
@set /a file1start=1
@set /a file1end=%file1start%+%file1%-1
@set /a file2start=%file1end%+1
@set /a file2end=%file2start%+%file2%-1
@echo "Convert two WAV files to Stereo. Select Left File Last."
@echo "WavStereoMerge <Left File> <Right File>"
sox %1 %2 -M "%~dp1%~n1-stereo.wav" remix %file1start%-%file1end% %file2start%-%file2end%
Save this as a file called WavStereoMerge.bat somewhere in your path. And put the sox.exe somewhere in your path (or put the full path to it in the batch file). You could put the sox exe and this batch file in the same directory somewhere if you don't have a common utility dir for adding commands.
This batch file takes two files as arguments and will create a new file in the same directory as the first file with -stereo added to the file name. When using the Windows Explorer the first file passed to the Send To is always the last file selected. So select the right file first, then the left file, then right click and pick Send To->WavStereoMerge
To create a Send To link with the WavStereoMerge command find the .bat file in Explorer and right click it and choose Create Shortcut. This will make a .lnk file called "WavStereoMerge.bat - Shortcut.lnk" Rename that to just "WavStereoMerge.lnk" and right click and cut it. Then in the Windows Explorer type shell:sendto in the box where the directory path is. This will take you to the location of your Send To menu. Right click and paste the short cut link you made there. This will add the new shortcut to the Send To menu.
Now go find two .wav files, select the right file wav, then the left file wav and right click and choose Send To->WavStereoMerge and you should get a -stereo file in the same directory that is a merge of the two files. This batch file handles stereo files for each part (it mixes it down to mono) or plain mono files or a mix even (that's what all the extra batch file magic is for). If you want to see the results of this you can add a "pause" statement at the end of the script so you can see what it does.
I don't think there is any way to add functions to the SONAR media browser to do this there, but it might be usable with the Utilities tool menu somehow. Still would be nice if SONAR had some built in support for merging mono files to stereo and perhaps breaking them apart too.