For those who wish to create advanced SmartObjects there is the abililty to create SmartObject associations. This can be implemented using the K2 Designers SmartObject designer. The following link provides the necessary information on how to create associations:

http://help.k2.com/HelpPages/K2smartformsUserGuide1.0.7/default.htm#SF_-_SODesigner_Configure_Associations.html%3FTocPath%3DK2%20Designer%7CSmartObjects%7CCreating%20SmartObjects%7CConfigure%20Associations%7C_____0

Please note that there is more information on the left pane of the landing page under the Configure Associations dropdown.