Hello,
Although the sticky is old I moved it to this sub-section because these laws are somewhat static.
1) Memory kits are binned in the density they are sold in.
2) When combined, we eat into the "guardband" put in place by the memory vendor to insure the best possible chance these will work at the kits rated speeds, voltages, and timings.
3) 6800MT in that density likely won't be possible. You will need to either tune the system manually or return both memory kits for a matched set.
13900KS / 8000 CAS36 / ROG APEX Z790 / ROG TUF RTX 4090