Hi welcome @IFW it’s nice to hear such realistic feedback.
Well, the reason is that Zynthian is not mean to be run on multiple displays.
Yet, on 10.10.2024 
Having the native Zynthian UI + the engines interface (or webconf or whatever) on a second display would be defintively cool for sure.
It’s still doable for adventureous: Zynthian is just an Open Source Linux system and, with a little surgery on the case, it’s +/- easy to add an HDMI output on the left side of the 5.1 kit.