i know its a common and frustrating thing to happen when no bus for long time then two bus at once or even the dreaded 3 bus at once, and maybe you've already thought through the problem but here are my thoughts from a driver's perspective
the main thing that slows buses down is loading and unloading passengers. that's not true, really--traffic, signals, reroutes, etc all add up and it's not the way i'd want to frame it (you wouldn't say "loading cargo slows down trucks" because cargo is the point of trucks) but maybe i can refine to say: the main thing that differentially slows down buses is loading and unloading passengers.
imagine for example bus A and their follower bus B (leader and follower are the common terms where i drive for the sequential relationships, it implies no hierarchy) on a 10 minute headway (scheduled separation). they have close to identical reroutes, signal conditions, traffic conditions (a 1am bus has very different scheduling than a 5pm bus, but a 10 minute gap usually isnt enough to matter). passengers accumulate at a basically linear rate. if the bus comes every 10 minutes, very few passengers are watching the schedule and showing up right on time. they just show up and wait
bus A, proceeding on schedule, picking up their allotted 10 minutes worth of passengers, encounters some minor delay which costs them 90 seconds. this could be anything--bad luck with a light, a passenger with a lot of luggage, someone parked in the bus zone, etc. nothing that an operator with their head in the right space would get upset about--common, very everyday occurence. but bus B by luck of the draw does not encounter this delay.
the buses are now separated by 8.5 minutes. bus A is now picking up 11.5 minutes of passengers and bus B has 8.5 minutes worth. the chaos of the universe can and often does intervene here to even them out, but sometimes it doesn't and 11.5 minutes of passengers takes longer to load and unload than 8.5. so bus A loses more time and and bus B gains time. the gap gets smaller and smaller, bus A gets fuller, bus B gets emptier, until bus B catches up. we have achieved bunching
so what do we do? bunching is an extremely unresolved problem. if you come up with a novel solution you should sell it to every bus agency on earth. but around here, as drivers, one counterintuitive idea is that the best thing is for bus B to get in front of bus A and build some separation. bus B, being empty, can go much faster (fewer stops, less time at each) and can pick up some downstream passengers for A and get A closer to their schedule. if B lingers behind A, both buses are slowed because A stops at every single stop and all the passengers get on the first bus they see (understandably and generally correctly) and you basically have one large and unhappy bus
knowing all this doesnt eliminate the wounded feeling of having your follower pass you and zoomacroom on down the line. but it helps
