I'm honestly 50/50 when it comes to this whole debate of whether or not there is a second Eeveelution this generation. My reasoning for both these theories are primarily based on what Sylveon's evolution method might be, also this is just my opinion so no harsh judgement. The first theory leans more to the side that there is only one Eeveelution this generation but because of that it evolves via a similar method as the last two Eeveelutions Leafeon and Glaceon which is to level up in a certain area. Now some of you may ask why? Well the three of them are very similar to one another in terms of their Japanese names...
1- Leafia (Leafeon)
2- Glacia (Glaceon)
3- Nymphia (Sylveon)
As you can see all three of them have a name that ends with the suffix -ia. But not only that they have a very similar design as well...
They have a boot looking coloration at the tips of their legs, kind of like the first generation Eeveelutions Vaporeon, Jolteon, and, Flareon all had collars and also shared an identical evolution method which was via stones. Now how will Eevee evolve into Sylveon if it has a similar evolution method to Leafeon and Glaceon? Well I'm sure some of you have already probably seen me post this once before but I'll do it again anyway, I think that Eevve will evolve into Sylveon if it is leveled up inside a Fairy Ring...
https://en.wikipedia.org/wiki/Fairy_ring
I can see this as a very probable evolution method to evolve Eevee into Sylveon.
The second theory is pretty silly and it just maybe has to do with my OCD-ish tendencies XP, but if Sylveon does get an alternate evolution method to all the other Eeveelutions then I'd beg to differ that there would maybe just maybe with the teeny tiniest glimmer of hope be another Eeveelution with an evolution method that is similar to Sylveon's.
Overall I'm staying true to my first theory and I'm also sticking with the majority of the community that there just might only be one Eeveelution this generation as much as it pains me to say it. But hey at least we got one, so no complains from me