When NXT moves to the USA Network on September 18, the show will run for 1 hour on USA and the second hour will stream live on the WWE Network. The same applies for the September 25th episode. This was announced via WWE's social media accounts.
For anyone wondering, this only applies to the first two weeks of shows. The entire two hours of October 2nd episode, which will be the first head-to-head show with AEW on TNT, will run on the USA Network. Starting with the 10/2 show, every show will air live for 2 hours on USA and the there will be a replay showing on Thursdays on the WWE Network.
From what I was told, the first two one hour shows on of NXT on 9/18 and 9/25 are meant to get viewers accustomed to watching NXT on USA.