Hello and welcome to the forums!
I would recommend updating your BIOS to the latest release version. You can get it from your motherboard manufacturer's website. Also, you may want to double-check your motherboard manual to see how the memory modules must be arranged in order for dual-channel to work properly. You sound like you know what you are doing, but it never hurts to double or even triple-check how you have things set up
In order for dual-channel memory to be enabled, you must use matched pairs of modules in the correct slots. So, as long as each set of memory you mentioned is matched (i.e., has identical specifications to the other stick in the set, which you indicated it was), then it *should* be able to work if you put it in the correct slots and set the BIOS options correctly. I am almost positive that the Kingston should work (it works in almost EVERY board out there), but the Elixir I am unsure of.
I would say this has to be a simple mistake of not setting something correctly in the BIOS/incorrect mounting etc., or something equally mundane. I would be willing to bet money that your motherboard is not experiencing any kind of malfunctions.
Hope all this helps.
AMD Opteron 165 CCBBE (2.7GHz @ 1.35v)
DFI CFX3200-DR (07/28)
Kingston HyperX DDR500 (3-3-3-8-1T)
Asus Dark Knight Radeon 4870 1GB
2x 74GB Western Digital Raptors in RAID0
500GB SATA-300 WD Drive
CoolerMaster RealPowerPro 850W
NZXT Zero full-tower case
Windows XP Pro SP2