https://www.asus.com/us/Motherboards/P8 ... sk_Manual/
On Page 1-16, Asus wrote:
You may install varying memory sizes in Channel A and Channel B. The system maps the total size of the lower-sized channel for the dual-channel configuration. Any excess memory from the higher-sized channel is then mapped for single-channel operation.
You should end up with 12 GiB available, as long as you select timings that work with both DIMMs.
i7-6700K, NT06-Pro, GA-Z170N-Gaming5, 32 GiB, RX Vega56, SM951, 5TB HDD, Blu-ray, FTZ01, SX800-LTi, C32HG70, RK-9000BR, MX518