Can you count? The A1200 has 2 ROM chips, and needs them both. This is because the 1200 has a 32-bit data bus, as opposed to the A500's 16-bit, so they needed the second ROM in order to work with the system's bus. If you plugged one of them into the 500, you'd have majorly corrupt data, as the second double of the word would be missing from every read.
The euivelent is if someone tore out every other page of a book.
