CRM 2011 - Adding two different entity type records to the same subgrid

I have two entities which I'll call entity A and entity B.  On another entity (entity C) form, I have a subgrid.  The subgrid is able to be configured so that entity A's records can be added to it, but how do I add records for entity B? There is no option out-of-the-box to add two different types of entity records to the same subgrid.  Any ideas?  Thanks