Fixed space between two dynamically sized blocks
I guess this is the proper way to have fixed space between to half-fitted blocks, but maybe later you'll have small issuer with container DIV, because child elements are absolute position and parent - relative, so it will not follow heights of child elements.  

for example: http://jsfiddle.net/XfStv/3/ 

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------UPDATE------

You can also use table property for objects, but it's not supported by IE 7:


here is example of tabled version code: http://jsfiddle.net/zur4ik/ehGRk/