Theroetically speaking: Both connectors have roughly the same specification, so they are capable of bearing same loads, withstand the same voltages etc. It doesn't make any difference which to connect or disconnect first, even arcing should be controlled likewise by creeping distances selected accordingly. Both connectors will ensure that the protective earth is connected first an disconnected last. The geometry prevents touching live parts for both. So there is no theoretical order for establishing connections.
Practically speaking: It's no good to use one of these as a switch. It will increase wear due to limited arcing. So concerning the design of devices connected via both of these connectors there should be another component like a switch which connects the circuit after plugging in both connectors and disconnecting the circuit before unplugging any of them. However as you can see for all appliances, there is no lockout mechanism to ensure a switching by proper switch or relay before plugging. It's nearly impossible to build such a switch.
Both plugs are meant to survive a plugging operation under full rated current (typically 16 A) and survive arcing in case of short circuits. However this is valid only if lines they are connected to are properly fused or equipped with breakers.
For your headline whether there's a standard or norm. A standard or norm is something which can be an obligation to either a manufacturing industry or trained personnel only. But both connectors can be found on household appliances so there is no applicable rule like standard or norm. If one had wanted to ensure a proper connection order by untrained personnel then it had to be implemented by technical means like some sort of lockout mechanism.