1) Have two tags: pack1 and pack2.
2) Assign posts to the corresponding tags depending to which pack they belong.
3) Protect the two tags in Tag Access Restrictions, for level 1, separated with a comma.
4) Create two buttons for level1, with pack1 and pack2 as custom capabilities for each one.
5) And then add advanced conditionals to single.php template to check the tag of each post, and verify if (current_user_can("access_s2member_ccap_pack1")) or (current_user_can("access_s2member_ccap_pack2")) to decide if I can display the post, or have to redirect to the Membership Options page. There will be some users that will buy both packs at some point, so in that case, current_user_can should be true for both conditions.
Beautiful. This will work just fine.
will they be able to bind the second pack they buy with the user account they created for the first pack?
Yes, you just need to create a Modification Button with s2Member's Button Generator. Supply both Capabilities in comma delimited format, and establish the cost for the upgrade. They will already have one Capability, so the new Button should include both Capabilities, and be sold at a lower rate.
The important thing to realize, is that s2Member is NOT yet capable of just adding a single Capability. You have to give s2Member the big picture, and include a full set of features in each Button Code. After checkout, s2Member will update the account to the new feature set; based on the Level and Capabilities you configured in the Modification Button.
This Modification Button should be introduced to a Customer that is already logged into your site ( possibly on the Login Welcome Page ), with a link that says " upgrade your account! ".