When the train is full the train is full there is no option for the second person :/ 

My suggestion take the reservation free regional train from Milan to Domodossola with connection further to Brig/Visp :)

Thank you very much. Sounds good. Good learning lesson for us!

These reservations are the backside of the Pass experience :/ 

@Mehrdad  is Visp the final Destination? Or where are you heading? Zermatt,Spiez,Interlaken?

Here a Reservation free Route from Milan (Attention its not Centrale) to Brig (the 2nd train goes further via Spiez to Bern) 

@seewulf No, Zermatt is the final destination. We have a short (30 min stop) at Milan (currently on the train from Florence). We will see if we can get another first class ticket.

if not, it seems we have to use the regional trains (an  additional 2-3 stops).

In Brig you can already change for the train up to Zermatt :) 

The Eurocities have the connection in Visp regionaltrains in Brig :)

Thanks, @seewulf ! Great point! Yes, probably that makes the most sense. Thanks a lot for taking the time and helpful replies. I really appreciate that!

Enjoy your trip and the nice stunning view :) 

@seewulf  thank you so much! Stay safe and well!