Bevendre

   Not a popular fanon, or even one that I’d ever heard of.  Given the implied age difference between them (Spitfire and Soarin are both portrayed as being at least a few years older than Rainbow, who is roughly the same age as Lightning Dust) it is nigh impossible.
   That being said, it is possible.  Magic has been shown to allow for time travel, albeit with limitations.  If Soarin’ and Spitfire were the foals of Lightning Dust, and sent back in the short period that the time travel spell lasts, then it could work assuming that the spell could be severed by the one who cast it.

Sorry I can’t defend this one very well, I have never heard anything like this one.