You can't transfer free credit between them. The idea of the MSDN credit is for Development and test (not production) purposes for the individuals who have the subscription, so things should be torn down regularly anyway.
There is such a thing as "Organizational Accounts" coming into azure (I think it's still in preview. Although that doesn't unify individual allowances, it does allow control over the individual's accounts.
http://msdn.microsoft.com/en-us/subscriptions/dn531048.aspx
Azure MSDN benefit details http://azure.microsoft.com/en-us/pricing/member-offers/msdn-benefits-details/