A few years ago the Swiss Railways introduced a little red plastic chip card that you can use to do things like put season tickets on, ski passes, reduction cars etc. This car is aimed at residents and to maximally confuse people they decided to call that card the "Swiss Pass".
And to further create more confusion they decided to link the account you create on the SBB website to this "Swiss Pass Account". So when you register on the SBB website you get automatically such a "Swiss Pass Account". If you log in you should have a account nr, in there somewhere. And the moment you buy a product linked to it they will send you the card. But for that you would need to upload a photograph first. One of the things you can buy this way is the Swiss Half Fare Card, which is valid for a year, and which is what residents typically get. (And personally, I think they should get rid of it and just halve the prices...).
But for tourists this is all not very relevant.
Here is the simplest thing you can do:
Just go to the RhB station in Tirano, the station where you will board the Bernina express. Go to the ticket office. Buy your ticket there (all the way to your actually final destination, it is never needed to ticket individual trains in Switzerland) and buy the Half Fare Cards there as well.
Alternatively if you really want to do this in advance you can buy both the HFC (the one for tourists, 120,- CHF) for you and your husband, and tickets from Tirano to Chur (lr to Luzern if that is where you are going on day one) on www.sbb.ch.
In Switzerland tickets are for travel, not for trains (in fact, there does not exist such a thing as a "train" ticket), they are always available in unlimited quantities, and you always ticket the whole trip, even if there are 10 different vehicles and companies involved... And tickets are flexible, not for a particular departure or service. That is why ticketing and reservations are done separately.
