If you use high precision gps, there is no need for more than one.
with low precision gps, theorically you could compute an average position, but they update their position when they want (you may have to wait .5 sec between gps A and gps B)... So you'll need to decode lat,lon,speed,heading, and more important, timestamp from each gps, add some serious maths, then recode output into a NMEA format......
If you're about to devellop a logic board to mix 2 lowcost gps units then you'd better have a look on "asynchronous / switched kalman filters" and build a high precision DR-proof board that would read your gps, a 2D accelerometer /gyrocompas and the speed signal from your car to output a highly reliable NMEA sentence.
Just my 0.02€