If we're allowed historical examples - until the formation of SELNEC PTE in the late 1960s, Manchester and its surrounding towns were well known for having many jointly operated bus services provided by the various 'municipals' of the time (plus the likes of LUT, NWRCC, . . .) The majority of the municipals had at least one route which ran into central Manchester/Salford (providing some colourful interest at city termini for those who like this sort of thing).
Pre-SELNEC Manchester numbered its buses with four digits, but all the other fleets tended to be numbered somewhere in the low hundreds. Taking a look at fleetlists from 1969 & early 1970s, there were certainly opportunities for the same numbered vehicles from different operators to be seen side-by-side (or at least in plain sight of each other)
For example, in the Greengate / Victoria Bus Station area, you might have seen Salford, Bolton and LUT vehicles all with the same fleet number (something between 150 and 300) waiting for departure. And along the busy A6 between Victoria and Irlams o' th' Height you could - in theory - see three identically numbered buses from these operators following each other one after another on different routes. (Unfortunately, Leigh Corporation's smallish fleet had low numbers and didn't extend to three digits, otherwise it could have been four of the same in Salford!)
With some magical alignment of the planets, the same numbered vehicles from LUT, Salford and Bolton might have been found at work on the Manchester - Bolton service 8 on the same day.
Salford and Bury Corporation had around 35 buses with the same fleet numbers, so you could plausibly get two of the same out together on the Manchester - Bury joint service (coincidentally numbered 35).
In Piccadilly Gardens, or east of Manchester in the Ashton-under-Lyne area, you might have seen three identically numbered buses from the SHMD, Ashton and Stockport fleets (somewhere between ~7 and ~50). South of Manchester there was a couple of blocks of overlapping numbers shared by Stockport Corporation and North West Road Car (though the NWRCC fleet would have been allocated over a wider geographical area than the traditional municipals)
Some of these multiple meet-ups would have been statistically improbable in themselves - but with a lot of buses running frequent services over a fairly long period of time, who knows what might have happened?