Unfortunately it is not currently possible to run both the systems at the same time, Unity also has the same issue.
The problem is that the XBoxOne has a limited number of resources for audio and when the game engine (UE4 and Unity) initializes its audio, it wants to use all of the resources.
This means that if they initialize before FMOD, FMOD will fail because there are no resources available, or if FMOD initializes first their audio will fail because they cannot claim all the resources.