I think it would be better to drill holes for the wires in the sister joists and run the wires thru them. an alternate would be to make the holes, then split the 2×8 lengthwise, running the cut thru the wire holes, sister the split 2×8 pieces to the existing joist using construction adhesive and 5/16 carriage bolts every 2 feet in each half of the split sister joist. this should give about 95 percent of the strength of a true sister with another 2×8. also spend the extra bucks and buy #1 clear joists so that there is greater strenght due to no knots. You will have to go to a real lumber yard to get the clear joists.