Originally Posted by Ajay73
...drilling a small diameter hole parallel with and tangent to an existing bolt, down to the lead...
I hope you meant to write 'iron' here. I'm not the expert, but I believe that the lead keels cannot have additional keel bolts added. They have 'J' bolts that are put in when the keel is poured.
I agree with the comment that this should not be done 'just in case', if there is no "Catalina Smile" there is no reason to sister the bolts.
My C25 has a small crack at the hull/keel join. I call it a "Mona Lisa Catalina Smile".