Years ago when I worked with my old boss, we use to install two 14-2 RX wires in one 3/8" connector. Some AHJ's would allow it and others wouldn't. they were never listed for more than 1 wire, but 2 wires would fit in there with no problem. This might be where the AHJ uses 90.4 of the NEC.
There was a proper two wire RX connector which you could install two 14-2 wires,14-3,12-2 or 12-3 wires. The newer plastic connectors ( I believe) are listed for multiple wire connections.
