Also I would manually tune the timings to the same specs across the board. If one kit has tighter timings than the other choose the looser timing for both. U may lose a tiny bit of performance, but will gain a greater chance for compatibility
You should make sure each channel has the same amount of RAM. Most motherboards are B|A|B|A. So it would go 8|4|8|4 or 4|8|4|8. Check your motherboard manual to be sure