You can modify the shaders on the fly by editing a text box, and clicking "compile". It should throw detailed alerts at you on any errors. It seems to continue gracefully after errors, although I'm not 100% confident in that.
So, I've been working on a talking head using morph targets. The interpolation had a couple of mistakes, so I thought it might be a nice opportunity to build in a web form for modifying the shaders on the fly. This would speed up development response even more than the rather impressive build-review rate we have already with WebGL. I was inspired by the interactive demo on Ricardo Cabello's website; http://mrdoob.com/139/GLSL_Sandbox. I wanted to use a "recompile" button, instead of the Google-style feedback per-typed letter. Anyway - it worked, and here is the demo. Now I can use it to fix up that talking head...
Drawing two separate objects with separate shaders is not working. It is incredibly hard to know which opengl functions to have bound before others. Official Khronos documentation is not written for humans. Current tutorials are murky and confused. So, back to basics - I need to start with a very simple example and establish exactly which commands do what, in what order, and which need to be called before rendering. Trial and error. Messy business. I had to do this with OpenGL 4, but it uses a different structure to WebGL's buffers and attributes. I think I can save a lot of community frustration by putting pen to paper after that...