Forum post 4760
talks about two programs (GPSGate and XPort3) that take one incoming data stream and multiplex it onto several ports. GPSGate works with serial or USB, XPort3 works with serial data only.
GPSGate specificly mentions the the Garmin Incompatability
and converting the data stream (click to read short comment) and says their program converts it standard NMEA (which, ironically, is a standard). What it doesn't make clear is if you can use the multiplexer to push more than just positioning data back and forth with programs that have data exchange ability (tracks, waypoints, etc), like EasyGPS or DeLorme. GPSGate is $10 (or less when on sale). You can pay more for more features, but the basic lite version is all you need.