Kindly note, you are referring to the Swiss Half Fare Card and the Saver Day Pass.
Swiss Half Fare Card – Switzerland at half the price | SBB
Saver Day Pass – book earlier, travel cheaper | SBB
Yes, you are eligible to buy the Saver Day Pass at half fare even though you are not yet in the possession of a Swiss Half Fare Card. You must be in the possession of it latest when you board a train or other public transport service. The ticket inspector will be interested in seeing your SDP as well as your Swiss Half Fare Card.
Have a good travel, kind regards,