To merge two account lists using tags:
Step 1: Use the List Action by going to Businesses > Lists > Click on the three dots to the right of the list > Select "Add Tags" and select the tag you wish to have added to the list of accounts > Select the Add Tags button to the lower right of the window which pops up.
Step 2: Follow the same step to add the same tag to the second list that needs to be merged with the first list
Step 3: Navigate to Businesses > Accounts > Filter the accounts with the help of added Tag.
Step 4: To export the accounts to a new List, click the Add to List button and follow the steps to create a list.