spoilers--->
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You put one set of compenents in one Urn and another set in the other. And you get a magic devise that has two uses based on the two components, one offensive, the other defensive like it says on the pillars. I dont know which urn is which though.
If you can de-stone the sphinx, he'll tell you about the machine, but I don't know what he tells you because I used the machine before I de-stoned him.
Sorry I don't have a total answer, but maybe someone else knows more if the sphinx doesnt tell you which urn is which.