When toolbar is too wide for a reduced width window (e.g. for two LO Writer side-by-side) the excess controls on the toolbar will be accessible from an overflow pop-up.  Click the ">>" icon.

You can of course use the 'Visible Buttons' droplist to deselect any of the buttons on a Toolbar to not display them and reduce the width. But you'll have to manually recover from that to restore (or simply restore default user profile).

Do not see a need to sub-group button actions on any of the toolbars to toggle them hidden or not.

Bottom line, there are plenty of means already to work with multiple module windows and customize the toolbars as preferred/needed.

-1 and this is a pretty clear => WF

Disagree, you can easy customize current default toolbars or create your own toolbars

-1 from me

I played around a bit with trying to throw away buttons and such. And yes I saw the double arrow but that got old fast as what I wanted was just out of reach.

So, in spite of all the ways offered to customize the toolbars by far the easiest solutions I have experienced in UI design of such apps over my many many years is as I have suggested -- fine granularity of chunks of functionality that can be enabled or moved around.

In my younger days I would have offered to contribute code to "fix" the problem but those days are behind me. In any case, it is a GREAT app and I greatly respect what has been accomplished.

Thanks
--jon

Besides the suggested solutions you can move the toolbar to the vertical side. We decided to make the TB not longer than 1280px [1] - but keeping it shorter means to either stack toolbars, which is ugly, or to limit the number of functions, which would be seen as regression by many users. 
And last but not least you may like one of the other UI variants (View > User Interface).

[1] https://wiki.documentfoundation.org/Design/Guidelines/ToolBar