The plans may be optionally (the word in the reg is "permissively") aggregated for coverage and nondiscrimination. If they pass separately, then they do not need to pass combined. If the plans are aggregated for either coverage or nondiscrimination, they must be aggregated for both. In other words, you have to use the same options for both coverage and nondiscrimination.
All plans of the employer must be aggregated for the average benefits percentage test.
If both plans cover a Key employee, then they are part of a required aggregation group for top heavy purposes.