To be able to handle live loads, we recently reinforced the 2×6 joists in our attic by sistering them with 2x8s. To achieve level, the 2x8s are not flush with the 2x6s on the bottom; they range anywhere from 1/2 inch to 1 1/2 inch higher.
The question is then how to apply the guidelines for drilling electrical holes. When they say to stay "2 inches away from the top and bottom" does that apply to both boards individually or just 2 inches away from the bottom of the 2×6 and 2 inches away from the top of the 2×8?
Update
Here's a cross-sectional view of the worst case scenario:
Holes are for wiring only; so 3/4" – 1" in size is all that's needed.
So the question is: How far down from the top of the 2×8 should I drill a 1" hole?
If I drill the hole so it starts at 4 1/4" down, this leaves 2" of 2×8 below the hole, but only 1" above the hole on the 2×6.
Best Answer
The rules for notching and boring joists allow you to
bore holes of diameter up to 1/3 the height of the joist, a minimum of 2" from the top or bottom of the joist
make notches in the outer thirds of the joist, width of notch up to 1/3 height of joist, height of notch up to 1/6 height of joist
The question and comments seem to say there is as little as 4" of overlap between the sistered joists, making it impossible to make bored holes that are 2" away from the top and bottom edges of both joists.
However if a bored hole is less than the 1/6 max height for a notch, it stands to reason the bored hole can be in that 2" zone; certainly a bored hole removes less wood than a notch.
Note that the height of a 2x6 is 5-1/2" and a 7/8" bored hole is less than 1/6th the height of a 2x6.
Therefore, as long as you
you satisfy the rules and run your wires without significantly weakening the joists. I'd aim for the approximate center of the overlap if possible, that should be the strongest spot to drill through.