Both (if happy together), will run at the speed of the slowest, though usually both are the same speed. You may be able to see this on the lable.
I don't know how you check their actual performing speed, Aida32 and Belarc only show their size and position. They are not likely to be a data bottleneck anyway. To use the fastest DDR RAM you may need a very recent 'screamer' motherboard