Are a couple who divorced from each other allowed back together intimately?
If they are divorced then they are not married, and they certainly may not be intimate. Halachikally they are two strangers, and all of the halachos of yichud, etc. apply. For cases where they are legally divorced but don't yet have a get, consult a Rav.
They may get remarried, unless the husband is a kohen, they got divorced because the woman committed adultery, or the woman married another man in the interim and he divorced her or died.