While there is not a direct "merge lists" button, there are a variety of ways you can likely accomplish your goals.
AWeber provides the option to export subscribers from a list and import them into another list without the need for the subscribers to confirm again. Additionally, there is also the option to include or exclude lists when sending a broadcast as well as sharing a follow up messages to other lists.
Exporting subscribers / Importing subscribers
If you wanted to move subscribers from one list to another, you can accomplish this by using AWeber's export function listed under the Manage Subscribers"section. First, select Currently Subscribed from the left hand segment options.
For the step-by-step instructions on how to export subscribers, please visit the following article:
Once you export the subscribers to a .csv file, simply import those subscribers into your desired list.
For the step-by-step instructions on how to import subscribers without confirmation, please visit the following article:
Including lists when sending a Broadcast
When composing a message from one campaign, you have the option to include other lists in the mailing. When including other lists, if a subscriber is subscribed to more than one of the included lists they will only receive one copy of the broadcast.
To include other lists for your created message, access the "Broadcast Settings" page. From here, check the "Include" box next to the list(s) you'd like to include. To include all lists, check the top "Include" box. Once complete, click the "Apply" button.
For the step-by-step instructions on how to include lists for a broadcast, please visit the following article:
Sharing messages between lists
Our "Follow Up Series Sharing" feature helps you share your messages, global text snippets, and custom fields between your lists. When you paste in a "Follow Up Series Sharing" code, any custom or global fields you had previously created will be overwritten. If you copy follow ups, any of those that you previously had in the list will be overwritten along with their related stats.
For the step-by-step instructions on how to use "Follow Up Series Sharing", please visit the following article: