i am strengthening a portion of a roof to support additional (50% to 75% more)dead load.  The existing "simpson" hanger capacity is ok for the new load.  i have seen others attach a new rafter against the existing rafter.  The member sizing is trivial.  How does the connection between the new and the existing work?  does the existing need to be relieved of its load prior to the 'sistered' reinforcement in order for the reinforcement to work properly?  is there an adhesive that is 'field friendly' for this connection?
 

I would design it like this:
Calculate the portion of the load carried by the new rafter. If they are the same depth, the load carried will be proportional to the width.
Determine the capacity of your connection. I would use nails installed with a nail gun.
Determine the spacing required to transfer the load carried by the new rafter.
Check shear and bearing capacity of the existing rafter under the new loads.

If it's all typical wood-frame construction (no brick veneer, clay tile roof, etc, the deflection under dead load will likely be minimal, and I wouldn't bother with trying to relieve the load before installing. What kind of loads and spans are you looking at?