Dual monitor - full screen fix / zoom

I have 2 monitors attached to my Mac mini. Both are the same model and use the same resolution. In order to use large documents over both displays, I disabled "Displays have separate Spaces" in Mission control. This works very well and I am happy with this.

If an application is requesting full screen access, this setup would expect either a full screen across both displays or a full screen display in the "current" screen. - However, the result is even different:

The display that "owns" the application goes to full screen while the other display turns black. This is the least useful option...

I learned that holding the alt key and clicking the green button at the upper left corner starts a "zoom" of the current window. However, this behaves very strange:

• Calendar and mail maximise the window to as large as possible in one display.
• Safari and Finder stretch the window to the top and bottom, however, the width stays untouched. If the window stretches over more than one display and is wider than one display width, the window is moved and shrinks to one display that shows the larger proportion of the window.
• Preview enlarges the window somewhat, leaving some space on the bottom and the right.

For some apps, this zoom option can serve as a work around, however, as every app behaves different, it leaves the user confused.

The ideal situation I expect for a full screen reaction:

• If the window is smaller than the width of one display: Maximise the window on the display that has the larger content of the window. The other display shows the app windows the same way as they were before.
• If the window is larger than the width of one display: Maximise the window across both displays
• In case of more than 2 displays: Maximise the window across the next larger size of displays it fits (i.e. if 4 displays are used and the app width is wider than 2 displays width, maximise the app to 3 displays.

Precondition for such behaviour is that all screens use the same number of vertical lines, ideally the same resolution. If displays use mixed resolutions, e.g. using a MacBook plus an external monitor side by side, would not create satisfactory results.

Any idea if there are work arounds or apps to achieve the behaviour I described above?