Welcome to FT!

What you're proposing to do will work, but there might be a better way to do it.

Rather than booking two one-way fares, look for an open-jaw ticket. This means you'd be able to book SFO-ZRH and then LHR-SFO as part of the same itinerary. If you book via someone like Travelocity (use the 'multiple cities' flight search option), they'll even be able to sell you the tickets on different airlines - eg Swiss for the outbound, and United back. Picking some random dates in March, I'm seeing fares that are around the same as an SFO-ZRH return.

If your itinerary isn't fixed and you're trying to squeeze the costs of your flight down, then you might want to consider doing London first, then Zurich: UK Air Passenger Duty for long haul flights costs more than equivalent Swiss departure tax.

You can then go and use a site like SkyScanner to look for a decent price between London and Zurich. There are a handful of airlines on this route, but bear in mind that easyJet and AirBerlin will charge extra for checked-in baggage.

(If you're feeling brave, you could use the 'multiple destinations' search to book all three tickets in one go; but that can push the price up somewhat - so it can be better to book the middle leg separately)