Apparently, Silabs EFRMG21 multiprotocol support could include Zigbee and Thread at the same time.
UG103.16 and UG305 multiprotocol docs say it is possible but have no Silicon Labs provided firmware.
"Zigbee and Thread are one example of suitable protocols for a concurrent multiprotocol implementation (not yet implemented in Silicon Labs stacks).”
“The device cannot be required to receive Zigbee and Thread packets simultaneously. However, because of MAC retries, this should not be a limitation in most use cases. The combined duty cycle of the Zigbee and Thread traffic to/from the device must not exceed what would normally be tolerated by a single Zigbee or Thread device.”
“A programmable multiprotocol implementation requires a chip with the infrastructure to support more than one protocol. Applications are developed in different protocols, such as Zigbee and Thread. Each chip is programmed with a single application in manufacturing. The customer achieves cost savings by being able to use a single chip for multiple applications and purposes.”
“Dynamic Multiprotocol makes use of the EFR32 hardware and the RAIL software as its building blocks. Zigbee, Bluetooth, and/or any other standards-based or proprietary protocols can then be built on top of these foundational layers, using Micrium to manage execution of code between different protocols.”