The memory controller in an Athlon 64 is in the CPU and the RAM compatibility rules vary a bit depending on which version of the CPU you have (Clawhammer, Winchester, Venice, etc.). You can find the version of CPU with
CPU-Z. I think you should be able to get two completely different modules working on any Athlon 64 by putting them in the same memory channel. That will force it to run in single channel mode. Your problems could come from trying to run one single-sided and one double-sided panel in dual-channel mode. On your motherboard I'd try the single-sided stick in "DIMM_B1" (the blue slot farthest away from the CPU) and the double-sided stick in "DIMM_B2" (the black slot farthest away from the CPU). If that doesn't work then you could try other slot combinations to see if both of the memory sticks will run together. Running in single channel mode will slow down your machine somewhat. You'd be better off running a pair of matched modules in dual-channel mode although you don't really lose all that much running in single-channel mode.