Merging Collections
You can merge the collections with each other using the merge feature which is located in the edit options of each collection.
Using the "Merge" feature
Navigate to the editing options of the collection you wish to merge and click on the Merge button.
In the merge window, select with which collection you wish to merge and click the Merge button.
This will merge all of the items from Car Rental collection in the Follow-up call collection.