Take a close look at the markings on the motherboard. Even though you see two banks of two slots, that may not be the way the computer is wired. My computer has:
1------2-------3------4
B1S1 B2S1 B1S2 B2S2

(B=bank, S=slot)

So if you are looking from left to right, the first and third slot are bank 1, the second and fourth are bank 2. Mine are also color coded black-blue-black-blue. Most likely the two existing memory cards are in the same bank.