Water bottle mounts
 

When did manufacturers start using two bottle cage mounts instead of one? Did it happen somewhat equally across all manufacturers or did some brands or countries hold out until later?

Probably whenever cyclists discovered that they needed more than just one water bottle.

Having two mounts is still not universal. Some mountain bike designs make getting in even one mount difficult.

There's also some new, three-bolt mount that I believe is motivated by bike-packing. I forget what that mount is called, but I've seen it crop up a few times.

When did they first become mainstream, regardless of manufacture?

They didn't.

I had a lugged Peugeot mountain bike in 1987 with them, and it was nothing unusual then. I then bought a tigged Diamond Back in 1989 with them, it was expected - i was a teenager, these were not big money bikes. So you're going back to the early '80s for starters.

Bottle cage mounts themselves seem to be a '70s thing, so twins were probably from near the start. Like someone said above, it's still fairly common to only have one, so twins as a mainstream feature don't really have their own landmark.

My '98 Cannondale T 700 had bosses for 3 bottles, with the third being on the bottom of the down tube. My current Surly LHT has the same. When I warmed up to the idea of hydration pack in 2000 I stopped using the down tube bottle bosses.

A lot of cycloss frames have only one cage mount.

Sometimes an external battery pack (DT) is reason for the manufacturer to forego a mount.
So I use a backpack or velcro bottle cage.

When my gum upgraded a year or so ago, I see all the spin bikes have room for 2 water bottles. I think it must be popular.

My mountain bike only has one water bottle cage mount on the frame. My road bike has two water bottle cage mounts, and the under saddle bag I use on the road bike has another spot for a single water bottle.

Dave

I did an image search of the 1982 Tour de France, and every bike seemed to have bottle mounts. So I did it again for 1972, and there seemed to be a mix of bikes with mounts, and bikes with band-on cages. So I would guess that they became standard across all road bikes on both tubes some time in the late 70s.