I have a Asus P6T X58 motherboard with 6 slots and 3 channels. In my manual, it says that due to Intel spec definition, XMP DIMMs and DDR3-1600 are supported for one DIMM per channel only. Your P55 motherboard has only two channels. Is your ram 1600? and, does your motherboard manual have the same caveat?
Dual channel is set by the bios if you have two sticks of ram, one in each channel. The extra stick will operate in single channel mode.