Over the years, they've "adapted" to dojo behaviors, but now their ukemi consists in maintaining complete control over the encounter in aforementioned fashion, while PRETENDING to be affected by nage. A cosmetic difference.... They allow the instructor to move at higher speed than they let "mere mortals" do it, and they don't growl at him when he uses atemi
From what you've written, I suspect that your teacher is high-ranked and highly skilled, and that the two students you complain of joined your dojo so that they can train with your teacher. At the same time, your teacher keeps them around because he (or she) wants -- and needs -- their style of ukemi to challenge him and enable his own aikido to advance. Those students may not "allow" your teacher to move at higher speed; it may be that they can't stop him from doing so.
If I'm correct, then the most your teacher will do is to advise these students to "cooperate" with other students, which may be the "pretending" that you describe.
Your teacher does not want them to lose the ukemi he desires. It's worth it to him if this results in the loss of a few students, while attracting new students who desire this type of practice.