At this point, I have added son and wife to my MDX. There are 4 of us going, my daughter and I (who have been since magic bands and MDX have existed several times), and my son and his wife, who haven't been for years and have never had a MDX. We have a resort reservation but no tickets yet linked to anyone. He will be getting military salute tickets in the next couple of weeks for us to link to each of us in our MDX. He has not yet created a MDX for himself.
In the future if he visits again, he will have to create one for himself or create one now. We will be together for this trip, and I have no problem allowing him to log into my MDX while we are at parks looking for 4th 5th etc fast passes. I just didn't know if it would be a problem for both of us to be logged into my MDX at the same time at the parks for instance trying book additional fast passes.
Also, for future, when he creates his own MDX for a possible trip without me, will he have trouble because I already had him linked to mine.
I guess it doesn't really matter when we do it as we will have to link our MDX accounts together whenever he does. I just don't want us standing in line after scanning our bands w our 3rd fastpass, and then each of us try to log in to my MDX to book something additional if this would be a problem. I have never tried that before so am unsure if 2 people can be logged into same account from 2 different devices at the same time trying to book additional fast pass.