I want to insert a separator between blocks and content, that are rounded with css3 border-radius property (see the picture below). What would be the best decision for separator make-up?

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You could apply a faux column approach to the parent element (#sheet?). I’m not sure if that will meet all of your requirements.
From a design point of view, if both pieces of content already are contained in a shape, would a separator really be necessary? Just seems like it would add extra clutter is all.