I can't imagine they wouldn't run simultaneously. The AHL is barely televised; if you really wanted to you could have the NFL dev league play Sunday mornings.
Well I think the whole point of this league would be for fringe guys to play and have a chance to earn playing time.
Guys like Vlad Ducasse (mentioned above) as well as 3rd string qbs etc. Who are backups, but virtually dont play at all whatsoever during the regular season but are still part of the 53 man roster
They wouldnt be available to play during the season (as they are needed as backups), so you would get an epicly bad talent pool if the league ran during the nfl season.
And playing on sunday mornings ?
I know theres a lot of hardcore NFL fans, but if you live on the west coast are you going to get up at 6-7am on Sunday to watch developmental players ?
The only way for a developmental league to succeed imo is if it took place during the non football monts of the year, which is basically march-july