Return the Nashbar saddle for a refund. If your bike is relatively new, see if the shop you bought from will take the Selle San Marco back and give you some store credit towards a different saddle. Or sell the saddle on eBay or Craig's List. Use the money from the refund and sale to purchase the saddle you want.

The other alternative is to swap your good saddle back and forth from one bike to the other. If both bikes have the same seatpost diameter, you can swap the entire post back and forth, which might be a bit easier. You can make marks on the saddle rails or seatpost with a paint pen or Sharpie to help with positioning.