I think you misunderstand which codec is a format and which one is an upmixer
DTS:X is a native pure mix with encoded height objects
Atmos is a native pure mix with encoded height objects
You cannot "upmix" anything to either format
The upmix versions of each are Neural X and Dolby Surround (DSU)
that can be applied ONLY to non native DTS:X or Dolby Atmos tracks
that are ALREADY playing all speakers available.
There is absolutely nothing to gain even if you could apply Neural X or DSU to a native DTS:X/Atmos track
....which is why it is not even possible.
The Neural X and Dolby Surround "upmixers" are only intended to expand:
DTS-HD, Dolby TrueHD, DTS, Dolby Digital, and linear PCM to use the extra Height speakers
in a home audio setup with 5.1.2, 5.1.4, 7.1.2 or 7.1.4 speaker configuration.
They (upmixers) simply don't work on material that already HAS height channel information, (DTS:X/Atmos)
Which is why I said: "There would be ZERO reason for anyone to want to apply DSU or Neural:X
to a native DTS:X OR Atmos sound track."
Don't make me draw a picture