I have a dual display (horizontal with 2 folder trees) and I want to maximize the active File Display (with folder tree). If I select the destination file display (bottom) and I press F6 (Set DUAL=toggle,remember), it maximize it the way I want, but if I press F6 again, my active file display is now the source (top). Also, I don't have 2 folder trees, but only one. How to maximize the active file display (with folder tree) and then return to the previous view?
Also, how can I define the folder tree width when I define the Dual Horizontal style? I know I can display/hide element, but how to specify the width?
When you "select the destination file display (bottom)" you are then making it the source. So the subsequent toggling OFF of the other display - then toggling the other display back on is sort of irrelevant. In any case, even if you used other means to get back to a single file display, it would naturally become the new source. The 'remember' arg to the 'Set DUAL' command just remembers the tabs and folde rpaths you had open before you switch off a dual file display - it doesn't remember the top/bottom or source/dest orientation.
I don't think there's a great way to get that orientation back automatically that wouldn't just work in the reverse way from how you might want it to work in other scenarios, but - you could certainly try using some additional commands added to your current 'Set DUAL' command... i.e. 'Go SWAP' to switch the file display top/bottom orientation, and maybe 'Set SOURCE=toggle'. But again, that might not ALWAYS be your desired result...
About the folder tree not being turned back on as well, the toggling of the second file display works independently from the tree... and I think using 'Set TREE' toggle commands along with the rest of the code you might end up with above might not work the way you want. If you think you'll ALWAYS want a second tree when in dual-display mode - then the Prefs->Folder Tree->Options->Open second Folder Tree in dual display mode option will be your friend here...
About the "Dual Horizontal" style question... aha, you've come across Styles eh? I don't use them myself... there may be some use of styles that may let you do some of the other stuff above in a different way. Not sure... but to your question, I was surprised to find that you can't save over an existing Style name. That seems odd... but, you can load the current 'Dual Horizontal' style, adjust the tree width the way you want it, then save that as a new Style (say 'Dual Horizontal 2'). From there - you may need to adjust a few things in Settings->Prefs for this new style... compare all the rest of the settings in the ~default 'Dual Horizontal' style to the new one you just saved - particularly the the Left and Right File Display settings/checkboxes - which will be different in the new style. I don't believe you can otherwise edit a styles settings in a way where you can actually view and edit the width numerically or anything like that, so I think you need to adjust "in lister" then save...