check out kd8cec video on his swr meter that connects to his standalone signal processor. I think that is the functionality you are trying to achieve?
The Nextion to sketch code is a little complex (basically kd8cec fills a buffer with commands and data and sends them to the nextion to do the work.) In the two nano configuration, both nanos are talking to the nextion which does the display at request of either nano. So you will want to look at the sketch in the “standalone signal processor” not the sketch that is currently running on your ubitx at the moment. Perhaps you can lift some code there.
You might also want to checkout how much memory you have left on the sketch you are currently running in you ubitx. I suspect that kd8cec went to the two nano configuration because he was running at of cpu power and memory.
Sorry I can’t be of more direct help here!