1. Never put single sticks in a dual channel MoBo.... a) you take a performance hit 95% in gaming, more in certain apps and b) you can never be sure of matching it up with another stick.
2. RAM is guaranteed to work only when you get them in the same package.
3. RAM manufacturers change things up over time, without changing model numbers... so the Model CX4567542LP today may have different timings and even different modules than one you buy 4 months from now. RAM vendor generally will buy from higher priced sources early in a models lifetime than switch to a lower cost supplier as production lines improve.
4. You could buy 2 sticks in different packages on the same day from the same even for and they could arrive int he same box from UPS and you'd still have no recourse under the guarantee.
5. Remember, RAM that is sold as 1 stick, may have been packaged that way simply because it didn't pass testing when paired with a 2nd stick.
That being said, that doesn't mean you can't get them to work.
a. Of course same manufacturer, same OEM modules and same timings is a good start.
b. You may have to bump the voltage a bit.
c. if ya can't do a. then set all to the lowest common speed, highest timings.
d. Regartding 3 above, sometimes you will