I am not sure if going forward with two versions of Angle can be done.
It has to be done anyway. OpenGL ES 3.0 contains many features which are, in combination, only found on about 80% of desktops and on about 0% of mobiles at this time. This will change, but slowly. So unless you want to see WebGL support (any version of webgl) plummet into the cellar, WebGL 1.0 backwards support is an absolute must.