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.
i7-8700K, H100i v2, RoG Strix Z370-G Gaming, 16 GiB, RX Vega64, Define Mini-C, SSR-850PX, C32HG70, RK-9000BR, MX518