It would get way too hot, they were just designed for 1 at a time. Try holding back the main beam switch, see how long it lasts, bet it won't be long.
I think someone on this site has done this, can't remember who though. get a couple of old bulbs from the scrapper and try it on them I've heard of the glue on the lens melting with uprated bulbs
I did it, but that was with rallye lights, with seperate elements for each light. I put a diode between the dipped and main switching wires, so that when you switch to full beam it holds on the current to keep the dipped relay on.