Tested an example with 2 iframes and it worked. Will test using multiple canvases in the same page too.

Removing Blink>WebVR component and assigning to Blink>WebXR 

Removing Blink>WebVR component and assigning to Blink>WebXR 

Reopening to add an integration test

See: https://cs.chromium.org/chromium/src/chrome/android/javatests/src/org/chromium/chrome/browser/vr/WebXrArSessionTest.java

This may be moot as the proposed AR mode would only allow one session.

Multiple sessions may be impossible given the proposal here: https://github.com/immersive-web/privacy-and-security/blob/d1a8ded7262e7d3d8c368fd57093a69b5fc53536/ARModeExploration.md