A mower should be able to take an extra set of blades, if not, WIMPY MOWER. An engine should be able to take the extra load from the blades, the engines should be made to take a load. Ask [email protected] about his engines and spindles-he's run his for 12 years.
It is sort of like asking, does it shorten the life of your truck pulling 2 lawnmowers more than if I only pulled one on a trailer? It may shorten it a bit, but not much. I now have almost 3,200 hours on a Kohler 22 that has powered double blades only. I've never used single blades on either one of my Dixie Choppers or the first 2 that I had either. I hope this helps.