where is the code to draw side by side scene

in graphicsEngine do_draw:

if (cull_result == NULL || scene_setup == NULL) {
// Nothing to see here.

} else if (dr->is_stereo()) {
// We don’t actually draw the stereo DisplayRegions. These are just
// placeholders; we draw the individual left and right eyes instead. (We
// might still clear the stereo DisplayRegions, though, since it’s
// probably faster to clear right and left channels in one pass, than to
// clear them in two separate passes.)

} else if (!gsg->set_scene(scene_setup)) {

so ,where is the code to draw stereo scene ?

When you create a stereo display region, a StereoDisplayRegion is created, but this points to two DisplayRegions (one for left and one for right) that are also added to the window. These do the actual drawing.