How can I enable a user to have multiple subscriptions?

I’m using the Membership plugin in a multi-site environment. I want to allow a user to have multiple subscriptions. Each subscription is a course of study. I need to allow them to subscribe to more than one course at a time. It looked like I should be able to do it. I went to a test user and added more than one access level and subscription. The access rules were defined using positive selections. But instead of the “positive” permissions being additive, they were the opposite, and the test user could not then access either of the subscription levels. I tried finding information on this in the forums and can not find anything that says this is resolved.