Under BC's provincial police firearm regulations, police are not permitted to carry pistols other than those meeting requirements of the regulations. Plus they need a chief constable's permission. So I'd say no, it doesn't happen. Why would you carry a "backup" that is the same size, etc as your primary. Most service pistols tend to be of the larger variety (restricted class) rather than tiny little prohibs, so they can be rather unwieldy in an ankle holster or elsewhere.
Many US agencies/departments have a list of approved non-departmental firearms that their officers/deputies/agents can carry, and it often includes revolvers too, so it would be more prevalent there.