I ran some tests to check how many newgens are being created with a second nationality.
For the first test I loaded every league from the following countries: England, France, Germany, Italy, Netherlands, Portugal, Spain, Brazil. Everything as playable, small database.
I chose the earliest possible start date for England, June 24th 2019.
There are a few thousand newgens generated right at the beginning of the game, even though I didn't use the "Add players to playable teams" option. This is normal, it also happened in FM 2019. I just ignored those newgens and focused my test on newgens which are actually generated from club's youth academies as the game progress.
The game was simulated until December 31st 2020. I then took a look at all newgens which were created from the first day of the year until that date. I ignored newgens created before, so I could have exactly 1 year worth of data.
Number of Newgens generated: 11974
Number of Newgens which had dual nationality: 1000 (8.35%)
I simulated another year and got this:
Number of Newgens generated: 6298
Number of Newgens which had dual nationality: 525 (8.33%)
The reduced number of total newgens generated is normal. It works like that so we don't end up with a insanely huge database. The game tries to stay within range of the total number of players you had when you started the save. As you can see, the dual nationality % is quite similar.
Now I needed to find out how many real players (meaning: newgens are excluded) had dual nationality. I went back to the beginning of the save and got those numbers:
Number of real players: 50994
Number of real players with dual nationality: 9939 (19.49%)
Keep in mind, as Andrew said above, that % could be even higher seeing how we don't have info on every single players possible dual nationality in real life. On the other hand, some of those dual nationalities were acquired by playing/living in the same country for a while.
Even so, you can see there is a big difference between real players and newgens dual nationality % numbers.
I decided to check if a different game setup would provide similar results. I had another save where the following countries were loaded (every league as playable): England, Italy, Portugal, Wales, Brazil.
Number of real players: 32595
Number of real players with dual nationality: 4819 (14.78%)
Newgens generated: 7244
Newgens which had dual nationality: 280 (3.86%)
Newgens generated: 4007
Newgens which had dual nationality: 238 (5.93%)
Newgens generated: 3258
Newgens which had dual nationality: 148 (4.54%)
As you can see, the % changes depending on which nations you have loaded as playable, but it's always lower than expected. If I have some free time in the future I will try to simulate a few individual nations (e.g. 1 save for england, 1 for spain, 1 for germany, etc...)
So, to finish my tests I decided to boot up FM19 and compare the numbers. I created a game with the exactly same setup as my first test. Those were the results:
Number of real players: 50510 (FM20: 50994)
Number of real players with dual nationality: 9505 - 18.81% (FM20: 9939 - 19.49%)
Number of Newgens generated: 11649 (FM20: 11974)
Number of Newgens which had dual nationality: 2320 - 19.91% (FM20: 1000 - 8.35%)
Number of Newgens generated: 5617 (FM20: 6298)
Number of Newgens which had dual nationality: 1183 - 21.06% (FM20: 525 - 8.33%)
As you can see, FM19 had more than double total newgens with dual nationality when compared with FM20. And, as a new feature advertised by Miles, FM20 was supposed to have a higher number than FM19. Something is definitely broken, and as Andrew said above, SI is aware and investigating. Real world simulation is important to me, so I will wait for a fix before starting a proper save.
I hope this post was helpful.