Make sure you are looking in the "Server Behaviors" panel, this is where you will find the Insert Record server behavior.
The "Behaviors" panel is where you will find Client Side behaviors.
mak sure that the first insert does not have a redirect set, and make note of the name of the session to be created.
Go to the bindings panel, and expand the session collection, make sure there is a binding there for the session created by the first insert. if not, click the plus button, select Session variable to create the binding.
you can then add the second insert behavior by clicking the plus button on the server behaviors panel, and selecting DataAssist -> Insert Record -> Insert Single record.
bind the relational column of your table to the session variable.