Actively? To the best of my knowledge, no. But the eligibility rules of the NCAA and NFL work to prevent a second league from gaining a foothold. Because leagues like the AAF cannot use college players, they are forced to rely on fringe NFL players and cast-offs. That means an inferior product, and a larger hurdle to establishing a brand and enough revenue to continue operations.
Meanwhile, college players that may have been eligible for a secondary/minor league, but not the NFL (age rule), won't realistically consider a secondary league like the AAF if it risks any potential future they may have in football. First, there is the instability inherent in an upstart league. As we saw with the AAF, a league like this can easily fold in under a season. Secondly, an 18- or 19-year old may be talented enough, and mature enough to play professionally, but playing for the new league would make them ineligible for college football. That means that if the league should shutdown, they would be ineligible to play at the college level.