Sounds like neither Multiviewport nor View Instancing (MMV) is not supported on Mac atm, and the logic in StereoAspects asserts. See attached log from a partner.
1) Enable Instanced Stereo settings in the project
2) Open it in Mac editor
Observe crash
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-174004 in the post.