RAM dual-channel question

On my motherboard I have four DDR3 slots, which are two dual-channels. I know to utilise dual-channel properly both RAMs need to be the same make, model, size and speed. However for the other dual-channel slots can I use a complete different pair of RAMs?

For instance, in dual-channel A I use 2 x Kingston 2GB and in dual-channel B I use 2 x Crucial 4GB. Will this be okay and safe to do for my motherboard/computer?