Render two WebglRenderingContexts in the same pthread

I want to render two WebglRenderingContexts in the same pthread.
`pthread_attr_t attr;
pthread_attr_init(&attr);
emscripten_pthread_attr_settransferredcanvases(&attr, g_pHwd1);`

the api could transfer one canvas to the pthread, so how can i transfer two canvas to the same pthread ?

the demo is in :
https://github.com/yangfangfang1204/offscreen