Hello Erikaaaaa
The CHF 30 voucher is to purchase an annual Half Fare Card for residents, not to be used for any other ticket purchase.
The Day Pass for 2 is primarily a promotion for residents, also to promote the resident’s Half Fare Card. Tourists can also benefit from the offer but the voucher is, so to say, useless, unless you want to purchase a resident Half Fare Card; this can be useful if you are a returning tourist and you do multiple trips to Switzerland in a year. The voucher does not grant any discount to purchase the Swiss Half Fare Card, what so ever.
Be careful when trusting on promotions that you have seen on facebook. They can be a scam, you would not be the first one to report a scam here.
Kind regards,
Roland