Hi Keith - there can only be one style assigned to each face. The problem you're running into there is that boolean union will automatically merge together coplanar faces into single larger faces which normally makes for cleaner and more simplified geometry but in your case here it's having the unwanted side effect of losing the ability to have different styles applied to each piece.

A couple of other ways you could do it is if the pieces touch each other you could delete the internal faces so you had 2 open surfaces that touched each other at a common edge and then use Edit > Join to glue them together, or you could draw in a line and use Edit > Trim to cut your face into 2 pieces after you do the boolean union if it's not convenient to use Join instead.

If that does not make sense could you please post the 3DM model file so I could take a look at how it's arranged?

- Michael