running them @ the lowest speed should be supported, are the timing and latency the same? Here a clip timing 3,4,4,8 and cas latency 3. Both pairs should have the same even if one pair is 1066, and one 533. Both will run at 533 but might cause hiccups if the timing or latency are diff.
Im gonna pop out and see if I can find something more concrete.
Hope this helped somewhat