We will be visiting in November and have only one day in Vienna. In the morning I'll be taking a walking tour of the Opera House/Imperial Palace/St. Stephen's Cathedral. I'm trying to figure out if we will have time to see the Sisi Museum and the Upper Belvedere Museum in the same afternoon. Thanks!
The Belvedere Palace has a great art collection and magnificent gardens. The two areas can easily take an entire afternoon by themselves. It’s also some distance from the Sisi Museum.
Yes, you can do it but will it be memorable or enjoyable? If you end your morning tour by noon and then head to the Sisi Museum (are you a huge fan of Sisi? If not, is this really worth it? It is Sisi overload) you could probably get in by 1 pm and then spend 1.5 hours. From 2:30 - 3:00 pm you will need to transit to the Belvedere. You get into the Belvedere by 3:30, which closes by 7:00 pm. You say, however, afternoon, so I assume you are getting on a boat? Perhaps you don't have until 7:00 pm.
So in short, yes, it is possible, but is it really worth it, especially if you aren't a Sis fan? Many other lovely things to see.