We will be taking a train from Zurich airport to Interlaken and then Zermatt to either Geneva or back to Zurich. Are those two trips for two people worth getting any kind of a pass?
Zurich airport to Interlaken Ost is 74 CHF, Zermatt to Zurich airport is 129 CHF, or 102 CHF to Geneva airport. Not enough to justify a pass or Half Fare Card.
What are you planning to do in Zermatt and interlaken? I assume your plan is not to just sit on your hotel balcony the whole time.
If you buy the Half Fare Card that will also give you 50% off on mountain railways and cable cars. So if you do plan to travel around the area than this should be included in your calculations on wether a pass makes sense or not.
And how are you getting from Interlaken to Zermatt?
From previous post, OP appears to be taking a hiking excursion from Interlaken to Zermatt.
https://community.ricksteves.com/travel-forum/switzerland/zurich-or-geneva-112a6723-5ea8-4218-b57d-b773c7708433
One way to save money is to purchase Saver Day Passes for your travel days well in advance, at least 60 days. Should cost you 52 CHF per person each of your travel days.
https://www.sbb.ch/en/travelcards-and-tickets/tickets-for-switzerland/day-pass/saver-day-pass.html
Should have added that we are starting a Backroads hiking trip in Interlaken that ends in Zermatt