In Canada, there is only one PIC on the aircraft. That person is the one designated as PIC and they are the one who signs the logbook. This person, and only this person can log the hours as PIC. If you have PICUS from another country, it is accounted for as copilot on your Canadian application and CV.
Canada does have a PICUS program, but it must be approved by TC for each company who wishes to use it. Each company will have its own program with very specific rules and it MUST be approved by TC. If the program is not approved, the hours are not valid on any application. Foreign PICUS is not approved!!!
On a twin otter, there are very specific CARS regarding logging copilot time. Copilot will be required on some trips (usually with passengers) and not required on other trips (cargo). If your company operations manual allows the aircraft to be operated single pilot (such as in cargo ops), then the F/O on that leg cannot legally log the hours. If you are flying a twin otter as F/O, keep very accurate records as to when you were required to be there. It is only those hours that are allowed on your ATPL application.
If you are operating as F/O on an aircraft that TC designates as a single pilot aircraft, then you can only log the hours if your company mandates in their ops manual that you are required to operate the aircraft with 2 crew. Make a copy of those pages to submit to TC to prove the hours you logged were legitimate. It is up to you to prove it. TC will not dig through company ops manuals to help you prove your hours, but enforcement might dig through to prove your bogus claims.
Remember - only one person is allowed to log actual PIC. If you are weaseling a way to log PIC, it will become very apparent on your license application and TC will tear your logbook apart.