the bulbs are two different sizes. you'd have to grind out a bigger hole in the reflector. and if you screw that up then you have to buy a new reflector, bake the HLA, open it up and replace the broken reflector and start all over.
if you want dual low beams the best thing to do is the dual headlight mod. there is already a low beam in the h4. you just have to hook it up to the switch. anything is possible. but the easiest and best solution is the dual headlight mod.