I just tried to @/at two people in one comment and I think the message reads rather weird:
I am notifying two people, but, this message just reads very weird to me. (Others think the same! I checked on Super User chat to make sure it isn't just me as I am not the best at Grammar!)
I think it should be changed to:
"Only one user can be notified at a time. The post owner will automatically be notified"
or, I think better
"Only one user can be notified by you. The post owner will automatically be notified"
What do you think?