From the receiving end of the needle, having two streams would likely double the chance of errors happening. Even the best phlebotomist has a bad day. Also, veins often move when you try to stab them.

Not to mention any blood-related equipment are single-use. So the needle, tubes, containers, they all go to a special bin (I hope to be recycled). Doubling up would unnecessarily increase the cost/effort.

I would highly recommend going to your nearest centre and learn by donating yourself! 😀