Doubling up IMO was largely driven by then Elite League teams blasting the Sky Sports TV money in the 2000's on very decent overseas riders they could never otherwise have afforded. British riders were largely never given opportunities to ride at that level like they had been before.
By the time the tv cash had dried up for good at the end of that decade, doubling up had started to take hold, by neccesity. No money was invested in the future of speedway, it was all blasted on trying to achieve instant success with most EL clubs having nothing to show for it.