how to create this? twist flow?

 From:  Matadem
11279.1 
Good day
I am trying to make a shape...honestly I do not know if it is possible.

Attached a pic of what I am after the 2 shapes need to join other following the red lines...
I tried by making a rectangle solid, flow to bend ...but when I try twisting it does twist the whole shape sideways as well.
Any way to accomplish this?
Tnx!
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