When we started with Civi 4.2, Membership Types within the same organisation were set to "upsell", ie if you had Membership Type X (OrgY) and opted for Membership Type Z (OrgY) then your single row of membership would change from type X to type Z.
This was most confusing in our use-case and we were delighted to discover that this default behaviour was no longer present when we upgraded to 4.5 (or 4.6, can't be exact here, sorry.)
I am virtually certain that we have observed non-upsell behaviour, ie, purchasing membership Type Z (Org Y) resulted in two lines of membership, type X and type Z.
However, at some indeterminate point in the last 12 months, this became no longer the case.
Now, it is simply not possible for a Contact to purchase a 2nd membership type within the same membership organisation.
In our case, it appears that if a Contact has a membership type X and then goes to create a new membership type Z within the same membership organisation Y, then Civi and Auth.net (our payment provider) will process everything correctly (online confirmation screens, payments correctly setup)... but NOT in the Membership area. A new membership Z will not be created, nor will the existing membership X be updated.
Am I missing something fundamental here? Or has a recent update in 4.6 confounded things?
We're on Drupal 7.43, Civi 4.6.14.
Thanks for your assistance!