It has easily been 10 years or even more when I used the extra buttons on the title bar to do window management on multi monitor setup, delivered by Matrox if I remember correctly. And here they are again, currently evaluating if DF would do what I want.

My setup is 2x 3840x1600, both in landscape mode on top of each other.
Windows is setup to have two monitors. I don't use the Surround option Nvidia offers since that makes a 3840x3200 desktop where the seam between the two monitors is the middle, is annoying when some application pops up there.

I have a very simple use case: I want to have a title bar button to maximize any window (which allows re-sizing) to my two monitors.

Looking at Functions I found two options that I liked when reading them.
"Span Window Across all Monitors" in two flavors, one toggle and one Force Span.

But both do not do what I want. I verified with a few applications such as Word. The default button "Move Window to Next Monitor" works like a charm, but the "Span..." act strange. It either moves the window to the primary monitor in Minimized state from the secondary monitor. Or when on the primary monitor it minimizes on that monitor.

I succeeded for a small part where I want one application always to be maximized on both monitors following this instruction from the df website: https://www.displayfusion.com/Discussions/View/launching-and-positioning-an-application-on-startup/?ID=a0f16c60-9c49-486f-a300-fe37589fb9d8

That does help, but not for the flexible switching where for instance Excel should be both monitors maximized or just a single one with the use of a title bar button.

So, am I doing something wrong? Is there another way of doing it?

I am using the latest version of Win 10, DF 9.3 (Pro Trial) and latest Nvidia drivers for the GTX 1080.

Ha, I looked around a bit and found a free utility with limited functionality but with still a lot of functions. I could enable a toggle keyboard switch which maximizes in more or less the same way, but with pixel origin, width and height. It does have an issue with the task bar on the left, my default setup.

Toggle in your program works by default with the Move Window to Next Monitor, just not the same key combo though.
I'll work with both for a while and see what behaves best and I like most.

The issue with the left task bar is the same in your program, the start x coordinate with this function is 0 + width of taskbar.
Since my taskbar is on the bottom left screen only, there is some not used space on the left on the top screen.
I played around with the other function that lets me open the window on coordinates ("Move window to specified location") but when I use 0 for x it starts the window at taskbar width. When I try to correct I can get it to have its left top more to the left but it seems the 100% is done before the top left since the width is not full screen width anymore, still the width of 100% - taskbar width.

Oh well, all luxury issues....
I am almost already used to the taskbar at the bottom of the screen again....

Thank you for the prompt response and solution!