Soda-88 :
He said both sticks work individually... they don't work together most likely cause they're running at different volts (1,5v/1,65v) and/or speeds.
Yes, it could be that but most RAM will switch speeds to the timing of the MB which is usually the lowest speed or default voltage, hell mine is a miss match of 4 different types of sticks, a 4GB, 2GB, 1GB, 1GB and I get all 8GB.
Or just be a picky motherboard/bad design. I remember back in the day, the fun of swapping RAM around and having sticks work fine in 1 system but not the other because 1 system didn't like double sided sticks, etc. They would work 1 at a time, but 2 together wouldn't because of the design of the chip. That's why I pointed that out, just could be a bad design or chipset implemenation.
But the OP didn't specify the brand of each chip and whether they are a matched set or anything, so it could be 2 different types.