Are you actually carrying two GPSr unit turned on at the same time?
If so, try using one to find a location and note the coordinates, turn that one off and turn on the other. See if they are closer this way.
Two receivers opperating very near each other, say less than about 2 to 8 feet depending on the receivers, can interact due to their IF stages interacting. Sometimes, I have seen one ham transceiver so desensitized by a common AM/FM radio that it was useless when both were turned on. Other times the effect is much less.
If they work okay when one is turned on, but not both, the answer is only to use one at a time.
If they give different readings when only one is turned on, something else is going.
Take both to a known location and see which is reading correctly.
Hope this helps!
My two cents worth, refunds available on request. (US funds only)
Dave_W6DPS