RadarOnline.com claims that BRITNEY SPEARS' first husband Jason Alexander is nailing KEVIN FEDERLINE'S first baby-mama, SHAR JACKSON.

--Kevin impregnated Shar TWICE before leaving her for Britney in 2004.  Earlier that year, Britney spent 55 whole hours married to Alexander, who was a childhood friend.  They got hitched in Vegas, then quickly had the marriage annulled.

--A source says, quote, "Shar and Jason were introduced several years ago at a gifting suite in Los Angeles and have been friends ever since, but a few weeks ago they began dating."