Transmitting devices inside the phone are the WLAN chip, the bluetooth chip and the communication with the mobile net. While talking or transferring datas the mobile net connection transmits much more than while only in standby.
This transmitting devices inside the phone are in a distance of only millimeters or centimeters from the built in GPS receiver of the phone. This may cause interferences and malfunction of the GPS receiver. Having another phone with you should have no effect if the distance is more than a few or 10 centimeters.
I was testing inside a light builded garden house in rural area.