Greetings!!!
We are very much excited to work on your project & assure you a quality service.
From your post we understand that you need a custom Joomla form that allows user to select email lists from several different constant contact accounts. We will use the Constant contact APIs and account access tokens(provided by you) to retrieve lists & send information.
The Account list will be made available with checkboxes & displayed, with array to define the account the list belongs to & next key would be the list ID.
Or Parse the incoming data by exploding the string value.
In either case, The two calls are made back via APIs, one for each account to add the users to that list.
With our skill sets like PHP, MySQL, Joomla, CodeIgniter, HTML, Ajax you can rely on us for quality service.
Hope you are comfortable with our proposal & looking forward for an early response.
Warm Regards
Suraj