In the Zürich metropolitan area the S-Bahn trains are typically 2+2. The long-haul trains are 1+2. Sometimes you will see that the S-Bahn train sets are being used for a long-haul route. This is generally announced as there is a train of a different configuration: 'In ein andere Formation', and the sign at the track will often say 'Erzsatzzug'.
I've not noticed if the online schedule notes if there is a substitution train, but I can imagine it does.
The newer 2+2 s-bahn train sets have been around as long as I have in Switzerland, over 10 years. But until recently I think they've been fairly limited to the Zürich and Basel areas.
2nd class seating apparently includes the stairs.
