It might work, it might not. Remember that any settings you apply such as clock speed and voltage will apply to both sets of RAM, so it's a question of "will they both run under x volts and y clock speed and timings?"
I've managed to run 2 sets of wildly different RAM together before. One was 2GB rated at 667MHz, 5-5-5-15 timings at 1.8v. The other was 2GB rated at 800MHz, 4-4-4-12 timings at 2.1v. To get it to run, I had to drop voltage to 1.9 and run at 667MHz. It turned out both sets would run under those conditions (although not entirely stably).