You should be able to search and link based upon both of these fields. You can set the search and target fields to be different on a search and link so use the temp field to look up the AC_ID field if needed. Can you send me a screen shot?
Just for completeness the issue was due to the way the search and link was working. The search on link on this field normally completes the manager field on the user record. In your system it was working the other way round so the managers were having employees linked to them but the quick action needed to be run from the manager record rather than the managed record.