The ability for multiple applications to share the camera was added in April 2018. Information on how to use this feature can be found here.

https://docs.microsoft.com/en-us/windows/mixed-reality/develop/platform-capabilities-and-apis/mixed-reality-capture-for-developers#simultaneous-mrc-limitations

Unfortunately Remote Assist does not make use of this feature, which means that applications which need access to the camera will not function correctly on Remote Assist calls. Most notably, applications that use Vuforia markers and 3rd party QR code libraries are affected by this.