If you have a motherboard that is compatible with DDR4 then you will be able to use 2400 and 3200 RAM together. DDR4 is still relatively new. Given this, not all motherboards will be compatible. Newer motherboards are compatible, but older motherboards may not be.
It is worth noting that while you can use them both together, it is likely that your PC will only use the slower speed rather than the higher speed. The majority of people will tend to use one RAM rather than two, but it is possible to use both if you prefer.