I recall trying this by first breaking the marriage between the two (in my case my wife & I) then trying to export her side of my tree.
Unfortunately I never did deduce a method of getting all of her relatives. I recall it would select all her ancestors going up the line, but not include dependents of those selected ancestors (eg, children of her uncle, etc).
I hope I've recalled that correctly. I was wanting to get all people on that side of the tree but never could.
Perhaps someone can tell us another method...