Horns 1 and 2 mix

The horn parts are reversing in play back the range. Horn 1 plays an octave lower than horn 2 (or horn 2 plays an octave above horn 1). Changing the octave result in unreadable and unplayable parts.
I guess there is an issue either with the mixer/Muse sounds or with the definition of the ranges in the instruments box but the latter would be strange that2 identical instruments tune an octave difference.

Any hints where the problem lies and how to resolve it. Thanks.