If both RAM sticks do work, but only one at a time, you have to consider two possibilites:
1. Mobo is defective - to test this theory, install your RAM like this: | _ and _ |. Then repeat the test with the other stick.
2. Mobo cannot support the combined density of your RAM sticks
or a distant third possibility is that the sticks are not compatable with each other.
Are the two sticks exactly the same?