Exclusive-hold (which is what you are describing) and non-exclusive hold (which is what you are asking for) is the result of a combination of Privacy settings in CUCM. Here is a link to the Feature Configuration Guide v12 section on Privacy that is worth a read.
Feature Configuration Guide for CUCM 12 - Chapter: Privacy
Unless your users actively use the Privacy feature the easiest way to allow for non-exclusive hold is to disable Privacy in the Call Manager Service parameters.
Users will need to press the line button, and then press the Resume softkey to pick up the call.
Let us know if you have questions or need additional information.
Maren