Have Eurail and Swiss pass, do I need both?

I think it unlikely that you will need both. Almost everywhere they duplicate. One particular place that the Eurailpass lets you down is in the mountains beyond Interlaken in the Berner Oberland. The Eurailpass is not valid on the trains to Grindelwald, Lauterbrunnen, Wengen or Mürren (although you get a small discount with it and use a day) whereas the Swiss pass gets you to every inhabited village. You then get a discount on the summit cable cars and trains.

Seems like a small benefit for an expensive duplication.

Can you return the Swiss pass (I don't know)?

Thanks for your response. I can cancel my Swiss pass (up to 4 business days before going) so that won't be an issue. I guess now the question will be Swiss pass for the time in Murren, Zermatt and Glacier Express or just the half fare pass?

One warning to you, last year because we had multi-country trip like you and we had a Eurail pass for those sections (as noted above you need a separate Bernese Oberland or other Swiss pass for that area). We have used Eurail many times before without a hitch—until this trip and I will NEVER use them again. We used the app which was a big mistake, we should’ve stuck to the old paper tickets. The app, at that time and unbeknownst to us before we left, was quite buggy. The problem is there is NO real time help from Eurail once you are in Europe. No phone number. They have an email but they reply they can only get back to you within a week! We could NOT get the app to properly load our trips. Finally after about 5 hours of online research and finding a traveler board where many others were having the same issue with the buggy app as us, one person posted a workaround he had discovered which ended up working for us (put all trips under a single title). But it was incredibly frustrating particularly because there is zero real time help. So, if you are using them I would recommend using old paper tickets and NOT the app. We only used them because we had had success every other trip but every other trip was paper tickets and because we had received a 50% discount.

The rule of thumb for European rail travel is ( with rare exceptions ) , Don't buy from third party re-sellers ( like Eurail ) . Buy your tickets from the companies that run the trains . I always buy direct ( DB -Germany , SNCF - France , OeBB - Austria , Trenitalia - Italy , etc. ) , never have had the slightest problem .