Membership - multiple membership levels canceling each other out

According to the Membership plugin instructions (under "Creating Membership Levels" about half-way down the page), I see this text:

When a user belongs to more than one level then their access rights are a culmination of all of their assigned levels.

I have a case where I need to utilize this, but it's not working for me.

1. I set up 2 categories -- "cats" and "dogs"
2. I set up 2 membership levels -- "meow" and "bark"
3. Inside the "meow" membership level," I add a positive rule for the "cats" category
4. Inside the "bark" membership level," I add a positive rule for the "dogs" category
5. I set up a WPMU user and manually add both membership levels to their account.

The result? The user doesn't see either the "cats" or "dogs" category.

If I remove either membership level, it works properly, and the user can see the single category.
(i.e. remove the "meow" level and the user can see the "dog" category
OR remove the "bark" level and the user can see the "cat" category)

But I need certain members to access multiple membership levels, in this case the user needs to see both the "cats" and "dogs" categories.

Am I doing something wrong?