"I want my members to sign up to more than one membership"
Each member can only have one membership at a time. "Multiple memberships" is a feature on our roadmap.
A possible workaround for now:
A membership can unlock multiple types of content, which means you can create "bundled" membership. This is great, but it's really only manageable if you have less than 3 memberships or a "master" membership that unlocks everything (i.e. 10 individual products + a master membership that unlocks everything).
Let's say you have two types of members-only content: Type A and Type B. If you want customers to access both types A & B you'll need to create membership C which contains both content types.