GPS signals are extremely weak - less than the ambient noise level.
GPS signals are "extremely weak" because they are modulated in direct-sequence spread spectrum (DSSS), that is why they can be under the ambient noise level. They are coded Code Division Multiple Access (CDMA). GPS was a military development and the system is designed to a high jamming margin against intentionally jamming. It is highly unlikely or impossible that one GPS receiver can unintentionally be jammed or blocked by spurious emissions of another GPS receiver on board.
I once used my GARMIN handheld GPS on board a A320 in mid flight to get some waypoints, but I had asked the captain before and he approved it.