You can either blueprint or copy mismatched ends, or rotate one end while the platform underneath is still ghost. In that case, when the platform builds out, it will connect mismatched.

Building a spaceship. A green underground belt placed on platform tiles shows both directions depending on which end you mouse over.

How to reproduce:

Place both ends of an underground belt in free-space. Either free-placement, or copy+paste.
Rotate one end. No connection will be shown between ...