Make two bootstrap menus collapse into one dropdown

I had the same problem, the solution is quite simple.
Add to your menus arrays such thing:

to the left menu

'container_id'    => 'navbarNavDropdown-left',

to the right menu

'container_id'    => 'navbarNavDropdown-right',

And you have just change your data-target to:

data-target="#navbarNavDropdown-left,#navbarNavDropdown-right"

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