Thanks Cliff,
Initially I thought your post would do the trick, but I think in the gantry config those arrangements are just for the mainbody so I'm not sure if they would affect the header area which I want this particular configuration to be within.
However, I did think of a workaround via CSS I could add...simply simulate the grid-11 and grid-1 (I've since changed my grid size from 12 to 16 since I originally wrote this post thinking it would help me)
#rt-header2 .rt-grid-10 {width:640px}
#rt-header2 .rt-grid-2 {width:40px}