It appears you can do that now. I just tried it with RIngCentral meetings and was able to do it.
If you are using an older version of RIngCentral Meetings:
After you start your meeting and click on Start Video (or have video already started), then click on
Share Screen. The normal sources of sharing will pop up and in the upper right hand corner will be an option entitled,
Select a Camera. This will activate the second camera you wish to use. See my attached screen shot.
After I created the screen shot RingCentral updated with a newer version. The newer version works IDENTICALLY the same as Zoom does, using the Advanced tab under Share Screen. Again, see the screen shot.