Some additional information:
Regarding the first two sentences of your query, for the sake of simplicity you‘d buy one return point-to-point ticket instead of two single tickets anyway.
Now just check the available ticket options for such a return trip on your intended day of travel. If a Saver Day Pass is offered as alternative to a return point-to-point ticket and its price is still reasonable, then get this one. Because it would allow free travel on almost the entire Swiss public transport network that day, or also as many times Zurich - Berne back and forth as you can manage…