Is there a way to enable shader pass from code?

I've got a sinple vert/frag shader for rendering beam effects. I'd like to be able to draw multiple beams in the same shader. Right now I do this by simply duplicating the entirety of the single beam code and tweaking some values for offsets. I'd prefer to be able to do this by (a) calling the same pass multiple times with different variables or (b) writing the shader with multiple passes and allowing code to decide how many passes to draw.