How to add space between curtain wall grids

[AidanHawkins]

Pleas see image and the 3mm of space between panel one and panel 2 (drawn in autocad which I am going to model

How can I add this space without making a mess with 2 grid lines and an empty panel between..

Is there any way I can create and 'empty' mullion or what is the process

[ToanDN]

Model a solid mass in place and cut the mullion with it to create the gap.