Can I Use Two Different Speeds of RAM in One Laptop? [duplicate]
Theoretically it's fine. Just make sure it has the same configuration (i.e. number of pins, size/design). This is very important for laptops.
Generally the faster RAM will be automatically 'underclocked' to achieve effectively the same speeds. You could attempt to overclock the slower RAM, but I honestly can't see there being much of an increase in performance.
It is recommended to have a pair of identical RAM sticks just for 100% compatibility, but as long as they are compatible, with specifications being mostly the same, it shouldn't be a big deal.