You still want a cheap mobile plan, so it's not technically cheaper until you have 4 people.
But that's comparing fiber to 10Mbps cellular, and even with 2 people the fiber is dramatically faster and only €20 more expensive. If you can live with 10Mbps then there is presumably some broadband option at no worse than that speed for less than €30 and then you're already ahead with 2 people.
And most people aren't going to be happy with speeds that low, but then you'd be paying more for each cellular plan.
In Norway there is no realistic option to have fiber cheaper than 45-50 Euro per month.
Based on personal experience 10 MBit/s is enough for video including full HD options. In fact it worked better than 100 MBit/s that was shared with several users and when somebody was downloading big gaming files.
But that's comparing fiber to 10Mbps cellular, and even with 2 people the fiber is dramatically faster and only €20 more expensive. If you can live with 10Mbps then there is presumably some broadband option at no worse than that speed for less than €30 and then you're already ahead with 2 people.
And most people aren't going to be happy with speeds that low, but then you'd be paying more for each cellular plan.