How can I follow "tag A + tag B" simultaneously?
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Im not 100% sure what you mean by "follow". You can choose some favorite tags, and any post containing those tags will have a yellow background (here is an explanation of all the colors).
Alternatively you can simply search the site for questions containing those tags using the search bar at the top right corner of any screen on Stack Overflow -
The search query for tags is simply the tag name surrounded by square brackets.
If you are referring to subscribing to tags - whereby you receive emails containing recent posts containing those tags, then I do not believe that you can subscribe to multiple tags simultaneously... You can subscribe to both and then perhaps utilize the tag filters on http://stackexchange.com/
Unfortunately, Stack exchange does not provide a way for you to restrict subscription ("following") emails to questions that have specific tag combinations. You may want to make a feature request for that, since it seems like a handy feature. (I'd use it myself if I didn't lurk in low-traffic tags.)