Try this in training mode - I am assuming you're on an arcade stick:
Hover your ring finger just over the HP button and your middle finger over the MP button. Bring down both fingers at the same time - you get MP+HP, and you'll hear both buttons being hit at the same time.
Do the same thing again but bring down your middle finger later. You will get HP then MP and hear two button presses.
Now, try reducing the amount of time between hitting HP and MP. Eventually, you should be able to do it where you hit the MP only a frame later than the HP. When you do this, you will get what is notationally written as HP~MP, which in the SF4 engine will input HP, then MP+HP on the next frame. MP+HP is the same as HP for normals, so you get two HP inputs on simultaneous frames.