A subscription form can be used to subscribe your visitors to multiple lists by filling out the one form. You must set up a subscription form in Pinpointe.
When creating your subscription form you can select which contact lists to allow users to sign up to. The user will then be presented with a list of checkboxes that they can select from to sign up to each list.
If you'd like to not give them the option but instead force them to be subscribed to multiple lists, you'll need to change the check boxes to hidden fields in your HTML code and make sure you "checked" them as the default value.
So for example (the difference is highlighted):