I appreciate your explanation, @ahmeto. I want to ensure that we're on the same page, are you referring to a transaction? Where you working on a present transaction and your colleague is updating/add some items to the currently running transaction is not possible. 

The multi-user function is allowing 2 or more people to work on the same company but can't work on the same transaction. 

This article will provide you the information about the features available in single-user mode and multi-user mode. Refer to the tables to get to know about the features that available and unavailable in both the modes so that you can select as per your requirements.: Features available and tasks you can perform in single or multi-user mode.  

Just in-case, I've add this article for your future reference: Hosting your company data in multi-user mode in QuickBooks Desktop

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