The first one, strictly speaking, only excludes overlaps of date periods (start/end date) with other date periods or single dates.

Any starting time stamp (as it may result e.g. from a combination of startDate and startTime) shall be lower or equal any ending time stamp (e.g. endDate) if both are given. Must not overlap with other <specialService> validity periods or single dates of the same enclosing <operatingPeriod>.

It does not exclude overlaps of two single dates.
Suggest to define two separate constraints.

Within the same operatingPeriod element, a date must not be contained in more than one specialService element. A date is considered to be contained in a specialService element if
- it is stated directly in a singleDate attribute or
- it is within the time span between startDate and endDate (inclusive in both cases)