Here's the problem - you can run the new, faster RAM at 5-5-5-15 . . . but you might not be able to run the 1.8V memory for long at 2.0V. Dunno. Your mobo requires 1 set of paramters to apply to all memory in the slots, which in this case would be 5-5-5-15 @ 2.0V.
Really should get a new factory-matched pair of sticks. There's a reason they are sold that way - to ensure that memory of identical rating will actual run in dual channel operation.
Additional question - I have a Micron 2 gig stick DDR 3 running at 10700 677 MHZ paired with a Kingston 2 gig stick DDR 3 rated the same. The voltage on both are rated 1.50 V but the frequeny and latency are slightly different. Both my bios and CPUZ tell me I am running 4 gig dual channel memory. Any reason to doubt that or replace one of the sticks?