Hello!

I need some advise to solve a current issue. Is there an extension that allows K2 item to have multiple authors?

That would certainly solve my issue, but if not, is there a workaround for the following?:

I need to enable only a specific user to view a list of items that may be created by multiple authors only for him and the original authors to see. Is there any way that I may do this?

Unfortunately, none that I am aware of.
Have you tried browsing the JED and KED?