My customers connect to me for support VIA "TightVNC" with a reverse
connection to a listening viewer. However I can only get them to
connect to the default listening port (5400). I tried changing the
listening port by changing the command to "-listen 5401" but then I
would have the viewer listening on both ports (5400 & 5401). How can I
change the code so it listens only on the port specified?

This is hard coded in the source code. I'm not an expert at it, but
I've encountered the issues that you've experienced with TightVNC
1.2.9. The code doesn't look at anything but the FLASH_PORT_OFFSET,
the value is located in rfb.h, and is referenced in the
VNCviewerAPP32.cpp, line 56:
			m_pflasher = new Flasher(FLASH_PORT_OFFSET);
			m_pdaemon = new Daemon(m_options.m_listenPort);

The only *good* way to fix it is to change the rfb.h to reflect, then
recompile vncviewer. Not exactly the best option if you don't have a
C++ compiler onhand. I'd post this as a bug.

The thing you can do, though, is use your firewall to block incoming
requests for 5400 and just receive on your specified port.

This is a free comment.

Have you disabled the AutoPortSelect setting?  Otherwise, it's likely
a bug in the code.  I could recompile WinVNC for you (using either
.NET or g++) if you would like, you could download the MinGW port and
do it yourself.  Hope to help.