If anything, moving your wife to the 2.4GHz may cause her problems due to more signal interference. But, it should not have any affect on her in game messaging.
Double check that both Apple TVs are updated, that the Zwift apps are updated, that the Companion apps are updated. Ideally, the auto-update does this for you, but I highly recommend manually checking for updates. Since December started, there have been updates for all of the above:
- TV OS: 5 Dec (ver 12.1.1)
- Zwift: 7 Dec (iOS ver 1.0.32268) – second update of Dec and probably a different version number for Apple TV
- Companion app: 18 Dec (iOS ver 3.0.2)
Then force quit everything (i.e., Zwift and Companion apps) on both Apple TVs and phones.