Rendering to multiple textures

To render to multiple textures, you still use the glDrawBuffers function. However, you use the enums from GL_EXT_framebuffer_object.

Example:

GLenum buffers[] = { GL_COLOR_ATTACHMENT0_EXT, GL_COLOR_ATTACHMENT1_EXT };
glDrawBuffers(2, buffers);

If a framebuffer object is bound with textures attached to GL_COLOR_ATTACHMENT0_EXT and GL_COLOR_ATTACHMENT1_EXT, they will both be rendered to now.

Rendering to multiple buffers

Rendering to multiple buffers, known as MRT to the Direct3D world, can be accomplished with the GL_ARB_draw_buffers extension. Using this extension is incredibly simple. For example, to draw to the back and AUX0 buffers simultaneously, you would use the following code:

GLenum buffers[] = { GL_BACK, GL_AUX0 };
glDrawBuffersARB(2, buffers)

And there you have it.

Of course, this is rather dull unless we can actually write different colors to different buffers. In order to do this, shaders must be used. In GLSL, you can select which buffer is written to by writing to gl_FragData[n] in place of gl_FragColor. If you are using GL_ARB_fragment_program, you can select which buffer is written to by writing to result.color[n].

References:

GL_ARB_draw_buffers specification