If a profile has multiple managers, they will all get the merge proposal. Does that answer your question?
By the way, I assume by "owner" you mean Profile Manager. (Not to be pedantic, but understanding the profiles aren't owned is important on WikiTree.)
If you see a profile with multiple managers it's possible that multiple profiles have already been merged together. The most common way to become a manager is to create a profile. When two profiles are merged, both managers become managers on the merged profile by default. Managers can be removed and added at any time, but that's less common.