Your account already contains an identity for you and one for daughter.
Son needs to create an
MDE account. That will create an identity for him in that account. He will then create an identity for his wife in that account.
You buy 2 tickets online from Disney. It will ask for your email address. If it's the same email address you use for your MDE account, the tickets will end up in your account automatically. You then go into your MDE account and assign a ticket to you and assign a ticket to daughter.
Or you buy 2 tickets elsewhere, then log into your MDE account. You enter the information for ticket # 1, then assign the ticket to yourself. Enter the information for ticket # 2, then assign the ticket to daughter.
Son buys 2 tickets online from Disney, and the tickets will end up in son's account. Son then assigns a ticket to himself and a ticket to his wife.
Or son buys 2 tickets elsewhere, then logs into son's MDE account. Son enters the information for ticket # 1, then assigns the ticket to himself. Son enters the information for ticket # 2, then assigns the ticket to his wife.
Or you buy tickets elsewhere for son and his wife, and give the ticket info to son. Son logs into son's MDE account. Son enters the information for ticket # 1, then assigns the ticket to imself. Son enters the information for ticket # 2, then assigns the ticket to his wife.
(I'm not sure how it works if you want to buy tickets online from Disney but have the tickets end up in son's MDE account.)
Completely apart from the whole ticket thing: after son creates his MDE account, he goes into his MDE account and sends an invitation to you; you accept. Or you go into your MDE account and send an invitation to him; he accepts. You do not need to have active tickets in your account in order to send and accept friend requests. Both of you check that you want to share information. Then you can book fastpasses for the people in your MDE account and the people in his MDE account. And he can book fastpasses for the people in his MDE account and the people in your MDE account.