There must be a lot of routes in London that pass other companies' garages, and a fair few that pass, or go close to, another garage of the same company (London Central's 171 until quite recently terminated on the premises of Stagecoach's Catford garage - not sure there are many arrangements like that left now.)
It used to be quite common in London for multiple garages to have buses on one route, although not always all on the same days of the week, as it was not uncommon for some garages to have a weekend only allocation on a suburban route, to balance the proportion of weekend work between garages. From memory, route 12 had 4 garages involved in to the 90s, and may have had more garages involved in earlier decades.
There is a tendency in the industry to concentrate routes at a single garage / depot, so that the local manager is responsible for the financial and operational performance of each route, rather than just responsible for getting x number of buses out every day, so the practice has become a lot less common, both in London and elsewhere.
The most geographically obvious garage in terms of efficient bus workings does not always lend itself to the most effective driver duty schedule - an obvious one would be a route that takes about 3 hours for a round trip is awkward if the garage is at (or close to) one end of the route, as doing 2 round trips doesn't really make up a full day's work, but doing 3 is too much unless there's a local agreement for a rota based on 4 long days a week.
I have encountered a couple of cases where multi-depot operation has gone wrong at the scheduling stage, with one or more journeys being left with no bus, and both depots assuming the other one does it. One came to light on the first day of a new schedule, another (a fairly obscure tendered service) wasn't noticed for a few weeks, and that service didn't last long after the county council found out...
Another one was somewhere each depot manager was allowed to make their own decision whether to run a Monday - Friday or a Saturday service during the weird bit between Christmas and New Year (or it may even have been during that area's traditional industrial holidays) and the two depots involved in one route each did different things. With the result some journeys didn't run at all, and some had two buses. Oops.