The purpose of "pairs" is to allow the memory usage the optimum ram scheme of DDR2/DDR3. When into gaming, this involves the "extreme end" of usage and thus the ram must be capable as well as the Dominator series is intended to do.
That website probably is offering just for sale and not following the true context of "best advise" for end use. In other words, you may have problems and/or these are really "recycled" ram sticks, no matter I just wouldn't trust that vendor provided the typical sale of such is usually in pairs. Do you feel comfortable knowing this? At least go in with open eyes.
I have visited some websites and found out that the corsair dominator comes in pairs always, but the place where I am going to buy them they come single in 1Gb is this normal, can I trust this.
ThIS is the info about the mems CM2X1024-8500C5D are these the same as TWIN2X2048-8500C5D but single ?