x2 stands the multiplication of the neural net with a factor of 2 from 22Mb to 44Mb. Rebel 15x2 is not meant as a new elo bomb but as a final touch of Rebel 15 and investment for a future Rebel.
Because of the double sized network (based on 2.75 billion positions) Rebel has much more chess knowledge available which can be easily demonstrated by playing a match on depth instead of regular time control. The match ended in favor for the x2 version with a 58% score indicating a 56 elo increase.
However there is an important drawback, because of the bigger neural network the NPS has dropped significantly, depending on your PC with 25% - 35% which has to be compensated by the better chess knowledge hence the elo gain is not much. On the other hand it makes Rebel 15x2 much more suitable as an analysis engine and an excellent starting point for creating an even better neural network.