This depends. When the same person/entity owns two abutting properties these properties may merge on title, meaning that they are now seen as one property in terms of the planning and would need to be severed in order to be sold separately. This isn't always a possibility if zoning by-laws or the official plan have changed the minimum lot size or frontage.
Not all abutting properties owned by the same person/entity automatically merge, but it is important to contact your attorney to ensure it has not merged on title.
Care should also be taken when purchasing an abutting property if it is your intention to at some time in the future sell one of the properties.