They will both clock to the lowest frequency. I'm not a fan of mixing and matching sticks of ram. In fact I always recommend that you buy a dual channel ram kit in order to have an optimal computing experience. On another note, make sure you're on the latest BIOS update for your laptop's motherboard and please pass on the stickered information for both sticks of ram.
Can you? Perhaps. Will it be improve your computing? Perhaps, perhaps not. Will I be better off buying a ram kit that meets my laptops specs? Yes!