I've a sketch on the front plane to generate a core feature. The result is an error message stating SW is trying to generate two cores. I've tried the same with unidirectional cut with the same error message. I thought it might be the multiple faces and tried to combine them but SW won't let me do that either. 

My goal is to move the yellow bodies up to the cavity and keep the center section (black) as a core feature.

Is there a better approach? 

Thank you.