I do not know the answer specific to simultaneous use of these specific devices over bluetooth. The Apple community seems to indicate that lately multiple bluetooth devices can be used simultaneously whereby earlier (circa 2011) there were some limitations around multiple sound devices. If we were in 2011, I would be wondering if "MIDI" is classified as a "sound" device (it shouldn't be, in terms of bandwidth required). In today's landscape I would say you have a good chance of these two devices working together over bluetooth - although it would be best to either confirm with another user who has the hardware or make your purchase such that returning for reasons of incompatibility is an easy option.

Yamaha may have more official guidance from their own testing.

Apple thread: https://discussions.apple.com/thread/2605727?start=15&tstart=0

Backing up a bit, however, there are other threads mentioning bluetooth as introducing latency issues when it comes to headphone monitoring. If the click track is not synchronized to anything (like your Montage tempo) - then the latency should not be an issue since it is simply a reference and not a click for synchronization purposes. However, if the click is not only supposed to represent a tempo - but also synchronized with the MIDI clock of your computer and/or Montage - then the time it takes for the signal to traverse through bluetooth to your headphones (latency) will be too much of a delay to serve as a useful synchronizing click.

It really depends on what your goals are for that click track.