what is the word that describes the sensation, in dreams, of being in two places at once??

Many people misuse the term "synchronicity" for what you have described, but what you have described is not actually synchronicity, but rather "bilocation" or "multilocation," a paranormal phenomenon attributed to several Catholic saints, psychics, and the occultist Aleister Crowley. It is usually applied in the context of an "actual" paranormal occurrence, where the subject is physically and observably present in two real physical locations at the same time, sometimes during a dream state at the original location, rather than both locations being part of the dream itself.