For dual channel - two identical modules are required. That's why they are sold in kits.
For best chance of ram modules working together without issues, get same make/model/capacity ram module.
So 16GB + 16GB.
16GB+8GB - will not run in full dual channel mode. It will either run in single channel or flex mode (part in dual, part in single channel mode).
Or you could return 16GB module and get 2x8GB kit instead.