Aweber has got email parsing and you can add subscribers when you email the details of ur lead to "listname at Awber" with a preset regex.
So you can set a custom .php thank you page and ask the first autoresponder to POST the subscriber data to the thank you page. From the thank you page, you can mail the subscriber details to aweber email parser, and the leads get added there. Make sure you set single opt-in for second list.
got it? i know its confusing..im sleepy!