Hi,
I would just like to know if buying a second DVB-T2 TV dongle would do the trick.
I don’t want to argue with my girlfriend during the World Cup 
Yes, you can use two dongles simultaneously.
If the programme your girlfriend is watching uses the same mux, you don’t need two tuners in theory, but this isn’t guaranteed.
If you are in the UK, you may also find BBC iPlayer and ITV Player are suitable for streaming the world cup if you want to save some money.
Sam