If I can get X power by doing Y. How am I not going to be able to get 2X power by doing Y twice? How could you possibly tell the difference between two people doing Y and one person doing Y twice?
The original paper from 1996, Rivest, Shamir, Wagner (see above) provides some nice real-world examples here.
1 woman needs 9 months to give birth to a baby. So 2 women will have twice the power and will need 4.5 months to produce a baby.
A bus full of passengers stands in the desert. They ran out of gasoline. The driver discovers that the next gas station is 50 miles away. This is no problem, since there are 50 passengers on the bus. They will just split the task and every passenger will walk his share of 1 mile.
The examples show that there are tasks where you do not get 2X power by doing Y twice. In my above posting there are references to mathematical examples in the literature.