Also, re:

> Try booleaning the main shapes.

So to answer this specific question - since those 2 main shapes are "joined surfaces" and not closed solids, it doesn't really make sense to boolean them together so don't try doing that. If you want these 2 surface objects to be glued together where their open edges touch each other use the Edit > Join tool for that job, not a boolean.

Booleans are meant for slicing up 2 solids and removing some material. You have neither solids nor do you want any material removed, so a boolean isn't the right tool for that, but Edit > Join is. The Join command only glues naked edges of surfaces together and doesn't try to intersect the objects and remove pieces like the booleans.

Actually though in your particular case you've got another problem yet which is the objects aren't even touching each other, it looks like you've removed a channel between them? There is empty space between them, they would need to at least touch each other in order to be combined:





- Michael