Framed multiple windows casing option

[mtldesigns]

I had no idea what the title should be..  

 

What I am asking is when we have multiple windows side by side, framed individually, what determines the casing material in-between?  I have a brick wall (both sides), and I want 6" in-between windows, but since the wall is brick, I get brick casing on two of my windows, the third is what I want.  Even adjusting to 4", I get the same result.  Exterior is visa-versa..  so.. I'm a little confused.

 

Thanks,

Edited 8 hours ago by mtldesigns