I often see Corsair recommending that you would need to run the memory at the next lower speed to have the combination work. XMP profiles for one kit don't apply when mixing kits.
Here is a FAQ from G.Skill, the memory manufacturer:
Q:
I want to install more memory, in addition to my existing memory kit. What are my options?
A:
We do not recommend mixing memory kits, regardless of brand or model.
By mixing memory kits together, there may be compatibility issues such as unable to boot or unable to operate at rated specifications.
Each of our memory kit are thoroughly tested to ensure compatibility within each memory kit. And because we have not tested our memory kit with your existing memory kit, we cannot guarantee compatibility when multiple kits are used.