I have done it. Single and only leaf spring on an Olathe drum chipper, broken right in the middle, too. It held for almost two years, until I sold it...still holding fine.
Prep your spring by grinding off all previous weld, and have good meeting surfaces. Pre-heat to 400°, then weld together with a very special rod: MG600. Yes, it is very expensive. Grind off excess and slag and imperfections, then post heat to 400° and cool slowly.
MG is the brand, and 600 is the rod number. This stuff is about $5.00 PER STICK. Yes! it is OMG expensive, but it does the impossible. Welded with all the right conditions it finishes at 180,000psi, which is almost double any other rod you can find. It is also a dream to weld with: it flows very easy and forms an nearly perfectly level bead on horizontal surfaces. The slag usually pops off without chipping, leaving a beautiful stainless surface. I have never had it fail to hold. Cast iron, carbon steel (leaf springs), Chrome-plated stainless steel hydraulic rams...it bonds well to any ferrous metal. So expensive, it is very hard to find. Worth every penny, if you get some, keep it hidden for your own personal use!
Note: it is very important that this weld have no imperfections when finished. If you have any "sharp points" where a bead meets the metal, a bead is undercut or sunken, or where the bead does not cover the crack...it will likely fail. These kind of faults act as a fulcrum for a new crack to form. I guess it goes without saying that the weld must not contain any buried flaws, either.