Multi-NUS on the nRF21540-DK

The application that you made with Multi-NUS should not change when you add front end modules like nRF21540. The application remains the same and what changes is the configuration of pins that are connected to your MCU and the nRF21540. 

Not true. There must be some added code to switch between tx and rx - that is, to turn the PA and the LNA on and off. Lacking such an adaptation of Multi-NUS, I could at least study the standard NUS application code to learn where in that complex and convoluted code such action needs to happen. Is there in fact some sample code for using the nRF21450? I couldn't find any.

Not true. There must be some added code to switch between tx and rx - that is, to turn the PA and the LNA on and off. Lacking such an adaptation of Multi-NUS, I could at least study the standard NUS application code to learn where in that complex and convoluted code such action needs to happen. Is there in fact some sample code for using the nRF21450? I couldn't find any.

SteveHx said:

Not true. There must be some added code to switch between tx and rx

Like I said before, there is no code change in the application itself but there are only build time configuration changes to enable FEM. The details of these are given here. If you read the details carefully, you will see that there are configurations to device tree and project configurations that are needed to enable FEM.  No logical changes to the application code are needed, only configurational build time changes are needed.