They are usually considered separately. Thai people, btw, consider mostly consider their country as part of both peninsulas; upper part in Indochina and Southern in Malay, culturally. Malaysia, however, is mainly considered as part of Malay Peninsula, along with Southern of Thailand. However, some people might consider only Laos, Vietnam, and Cambodia, all are ex-French colonies, as Indochina, and some also includes Burma. Please be noted that this is how Thai people think, not the technical definition.