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:
How do I export my subscribers?
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:
How do I import my subscribers without confirmation?
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
You can easily move your campaign from one list to another by going to the "Campaigns" page and clicking "Copy to List" underneath the campaign name. Please note: when you copy the campaign, it will be in the draft state in the new list. You will have to activate it before subscribers on that list receive it.
If you're using a follow up series, you can move the series to a new list using the follow up sharing code. 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.
Sharing Landing Pages Between Lists
You can also move your landing pages. In order to do this, go to the "Landing Pages" tab and click "Copy to List," underneath the name of the landing page. Please note: when you copy the landing page, it will be unpublished in the new list. You will have to publish it in order for it to be active. When you publish the page, it will have a different URL associated with it in the new list.
And that's it! Congratulations, you have successfully merged your subscribers, messages, and landing pages. Now, you can work from one list and utilize tags to further target your subscribers.