Hello Cheereeshu53
If I do not have the Swiss Half Fare Card,
I can not purchase “Day Pass for 2”.
Correct. The conditions for the Day Pass for 2 are:

Depending on your travel plans (e.g. you gonna stay a few days) the Swiss Half Fare Card could be a great investment. Purchasing full fare tickets can get very costly especially if you plan mountain excursions. If you are going to spend more than CHF 300 on tickets per person, you are break even and the half fare card pays off. This just as a small side note.
Have a great day ahead,
Roland