You have a query that returns 99.9% of what you want. All that is needed is to subtract the 2 weights.
select *, two.ITM$Weight - one.ITM$Weight as WeightDiff
from etc . . .
NOTE: Your expected output contained an error in the WeightDiff in the 2nd row.
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How to Post to get the most: http://www.sqlservercentral.com/articles/Best+Practices/61537/