Two ceiling lights on one connection

Slotting, putting in cables and plastering it up again

This is the most elegant solution.

and will be done in an hour...

When you borrow the right tool for slotting in reinforced concrete.

If you borrow the right tool for chasing in reinforced concrete.

There will probably be plaster on the ceiling - a screwdriver and some firm scraping will almost suffice

It's written above, concrete.... We also don't have any plaster on our ceilings, they are smooth anyway (because of the prefabricated parts in our case).

If someone asks me about our ceilings, I also say "concrete". Even with prefabricated parts, plastering is very often done instead of just filling the joints.

The OP should get in touch about that.