You posted your question in the German-speaking SBB Community, while the topic has actually already been discussed in the English-speaking SBB community:
[Info] Special offer: the Day Pass for 2 for CHF 79, until 23 October 2023
The Day Pass for 2 is NOT available for holders of the Swiss Half Fare Card.
Normally, in terms of reductions, the Swiss Half Fare Card and Half Fare Card offer the same advantages. However, public transport companies are looking to expand their resident Half Fare Card customer base with this Day Pass for 2 promotion. It is therefore a marketing initiative and not a regular ticket offer.
Thanks to Saver Day Passes you can still benefit from inexpensive travel on Swiss public transport.