I think that "against the rules" is a nice way of putting it. It would be stealing.
Sometimes things that are against the "rules" are not quite as bad. Sometimes they're only against the airline's rules, but not illegal. I don't know, but maybe buying a ticket with extra segments that you don't intend to use because it comes out cheaper that way is against the airline's rules. But I don't think it's illegal, and it's very hard to find fault in taking advantage of such a situation.
But in this case you're ending up with benefits of status that you did not earn, or taking an award from them using "combined" miles when they're not willing to give you the award on such terms. I think that you essentially would be "stealing" that award, or the benefits you earned through status.
I happen to have a cousin with the exact same name as me. I fly quite a bit. He flies occasionally, and not always on the same airline. I suspect that he hasn't even bothered signing up to any programs. It's always felt like a waste to me for his miles to go to "waste" sine it would be so easy to have them credited to my account. Just like it would be so easy for me to get reimbursed by his insurance company for Doctor's visits I've had where he lives.