Answer

Silicon Labs does not specifically have an antenna diversity reference design for EFR32MG.  However, you can reference the EM35x antenna diversity design for an example of external RF switch that could be used.  As of the EmberZNet 5.7.4 stack release, transmit/software diversity (switching antenna path when MAC-level Ack is not received) is available in the EmberZNet stack and any GPIO(s) can be used for this antenna select function, based on selection via the Antenna plug-in.  Receive/hardware diversity (assessment of RSSI on incoming packet to select strongest path) is not supported.  That being said, GPIO selection per MODEM_ANT0 and MODEM_ANT1 (in the case of a 2nd antenna select GPIO, as with an external RF switch) in your design will ensure that transmit/software diversity can be used in the short term, with a software upgrade to receive/hardware diversity at a later date, if available.  See EFR32MG1 datasheet Table 6.7 for details on GPIOs available for the MODEM_ANT0 and MODEM_ANT1 functions.

In terms of hardware-level recommendations, the antennas should be separated by at least 1/4 lamba distance and should be rotated 90 degrees from one another.  Please reference the EM35x antenna diversity design noted above for an example of this.