I use Street Atlas 2006, and the yellow Earthmate receiver it comes with. To split the GPS, I use the
free Delorme Serial Emulator . Here's what I do, to make sure there are no problems.
Install the Delorme Serial Emulator.
Make sure the Emulator is not on Auto-Start (it will always screw up, if it's on this for some reason)
Set the Emulator to "Off"
Start Street Atlas. (I have 4 USB ports, and using my rear left one for the GPS)
Get a 2-D or 3-D lock in Street Atlas.
Right click the Satellite icon on your system tray and turn to on. (Or just start the emulator however you wish). It should say "The emulator is now attempting to acquire a 3-D fix"
A green dot should appear letting you know it's working.
Right click the satellite dish icon again, and choose "Ports" this time.
Select all of them as NMEA.
This is the tricky part, as I have had to mess with it to find the right ones, but I use COM's "2, 5, 6" in that order. (Having the GPS connected to the rear, left USB port)
I then open GR3, and hit "Start GPS" and a list of COM ports open, I have this one set to "COM5."
Open SpotterNetwork, File>Settings> and Select COM6.
Walla, works every time as long as I remember to put the GPS unit into the same spot as the time before.
Edit: I guess I should mention if you have Windows Vista, you may run into problems, but Ryan McGinnis has found a solution in this thread....
http://stormtrack.org/forum/showthread.php?t=15831&page=2