I don't know for previous version, but in WAP R2 Preview, you can share a subscription between multiple tenants (ie accounts) exactly like you would in WindowsAzure.com. You simply need to assign co-administrators to the subscription. This can be done both in the Tenant Site and the Admin Site.
In "my account", click on the Administrators tab. Then you can add new co-administrators to any subscription of your choice. Moreover, you can even assign as a co-administrator an account which is not "yet" signed up.
Also, only the principal owner of a subscription can share their subscriptions: co-administrators cannot give co-administration rights to other tenants.
- Proposed as answer by Charles Perreault Thursday, October 03, 2013 12:36 PM