...

void ofApp::draw()
{
	ofEnableAlphaBlending();
	
	//pass 1
	glEnable(GL_CULL_FACE);
	glCullFace(GL_FRONT);
	
	shader.begin();
	
	mesh.draw();
	
	shader.end();
	
	//pass 2
	glCullFace(GL_BACK);
	
	shader.begin();
	
	mesh.draw();
	
	shader.end();

	//disable culling
	glDisable(GL_CULL_FACE);
}

...