First the note: Did you merge the two tubes?
Second: Red lines are an invalid poly. If you dont select the single vertices counter clokwise.
Select both tubes in object mode.
press ctrl+shift+M to merge them to one object.
The select two(!) vertices of each tube, where a new surface should be created.
Select those 4 vertices counter-clockwise (thats important)
Then create new surface using ctrl+shift+S
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