cutting a power plug
Jakob, the big problem about that master/slave extension cord is not its plug. It's its sockets. They accept the Danish Type K earthed plug which means you can put earthed devices into them.

The manufacturer of these devices with earthed plugs use them by purpose. Their equipment has no double insulation and when a fault between live and the metal case happens, they need the earth wire to trip the circuit breaker immediately. So you are safe.

But your master-slave socket does not have an earthed plug. It does not have an earthed wire to trip the breaker. That means a fault at a device plugged in will electrocute you.

This master/slave socket you have there is inherently unsafe and should have never been produced.

You can turn it into an electrically safe device by either rewiring it to all-earthed (replacing the cable and most likely also the wiring inside) or stuffing the holes intended for earthed plugs permanently (securing plastic prongs plugged into the earth holes e.g. with 2-component epoxy) so noone could ever plug in devices which need an earth connection in there. Then you may also cut the plastic of the plug so it can be placed into a schuko socket.

I would instead send this insane piece of shit to the local RAPEX official along with information who sold it. So they can seize the whole charge. And hopefully no one gets electrocuted until then.