We created a series of custom fields in Marketo for each of our different newsletters. You'd use a smart campaign to listen for the double opt-in confirmation for each of the newsletters, then when triggered you would set the value of the newsletter specific field to true. You could then use those fields to determine which newsletters they should be receiving. That would be the longest route.
Another option would be to only require double opt-in for the first newsletter they signup for then if they subscribe to the second later on since they're already opted into receiving email from you you just subscribe them to the second newsletter.