If you pull the tube 2 fabric forward and align the edge with the 5 cm line beside the slit (letting the tube fabric overhang the slit) then it looks like it will lie flat. You don’t have to force the existing corner to sit on the slit – you can trim the forward edge of tube 2 later so it lines up with the slit.
Does that help? I’m not 100% sure I understand the issue.