IIRC Voyager 1 traveled faster than Voyager 2, so it quickly passed Voyager 2. They named it Voyager 1 because it was going to arrive on target sooner.
From Wikipedia: "Voyager 2 is a space probe launched by NASA on August 20, 1977, to study the outer planets. Part of the Voyager program, it was launched 16 days before its twin, Voyager 1, on a trajectory that took longer to reach Jupiter and Saturn but enabled further encounters with Uranus and Neptune.[4]"
I watched the documentary The Farthest (really good, highly recommend https://www.imdb.com/title/tt6223974/) on Netflix a few weeks ago. I might be misremembering; feel free to fact check me.
Thanks. This bit from Wikipedia doesn't explicitly confirm, but it makes your answer very like:
> Two trajectories were selected. One was designated JST: its mission would take it to Jupiter, Saturn, and Titan, with the probe's trajectory designed to optimize the Titan flyby. The second was designated JSX: it would be launched on a trajectory that would preserve the option of a Grand Tour, while serving as backup for the first probe. It would arrive after JST, and if JST were successful, it could continue with the Grand Tour. If JST was unsuccessful, JSX could be diverted to perform the Titan flyby itself, which would eliminate the possibility of a Grand Tour.
> The two spacecraft that launched retained the same mission concept. Voyager 1's course was optimized for the Titan flyby and Voyager 2 for the Grand Tour. Voyager 2 would reach Saturn nine months after Voyager 1, giving plenty of time to decide if it should proceed with the Grand Tour. Additionally, by launching Voyager 2 first, Voyager 1's launch could be re-targeted to perform the Grand Tour if Voyager 2 were lost in a launch failure. An option to skip Voyager 1's Titan flyby and proceed from Saturn to Pluto was identified, though Titan was still considered the more interesting target, especially after images from Pioneer 11 indicated a very substantial atmosphere.
> With Voyager 1's mission complete, Voyager 2 was cleared for an extended mission to Uranus and Neptune, fulfilling the goal of a Grand Tour as proposed in 1964.
Well that’s unusual isn’t it? Perhaps there was a temporary issue/delay with #1? I didn’t see the explanation in the article.