You can merge companies from multiple collections into a new list using Advanced Search:

Start by selecting Add company criteria -> Collections contained in include.

You will then be able to select the collections you’d like to combine from the drop down list.

By default, this option is set to any, allowing you to merge multiple Collections together, but you can also specify that companies must be included in all of the Collections selected.

In addition, you can choose to exclude companies that appear in a Collection by selecting none from the drop-down menu.

Finally, click Search for companies to return the desired list of companies.

This updated list can then be saved to a new collection using the Add to Collection button, or quickly saved and re-loaded using the Save search button: