That’s a good question, though I don’t see an out of the box solution. If you are using the camera viewport rect property to have your two cameras render one after the other at the same window.
One solution would be to grab the output of your camera as a render target and create an HTML image which you can use in your DOM. From there you can style your image as you like using CSS styles. Though I am not sure what would be the performance hit in this case, might be a bit slow.
Here is an explanation/example on how to do that: