So, we've now had a bunch of info. Revice will indeed feature two Riders form the start. Kamen Rider Revi and Kamen Rider Vice.
Within the show, an organization called "Dead Man" is trying to summon demons hidden within human beings. They use items called "Vice Stamps" to draw them out as monsters called Deadmen.
Our protagonist is Ikki Igarashi, played by Kentaro Maeda. He makes a "deal with a devil" - specifically the one within him, called simply Vice. When Ikki transforms into Kamen Rider Revi (the mostly pink and blue one) Vice gains a physical form called Kamen Rider Vice (the mostly black one).
It seems that when Revi uses the Vice Stamps on his belt, he will not change forms himself but instead it causes Vice to change. He is already confirmed to have forms based on a T-Rex, Megalodon, and a cactus!
The series will also feature Ikki's brother Daiji and sister Sakura, played by Wataru Hyuga and Ayaka Imoto. Joji Karizaki, the developer of the show's Rider System, is being played by Hamao Noritaka . A Dead Man executive named Orteca is played by Hayata Seki.
The final bit of casting news has proven rather controversial. The voice of Vice will be provided by Subaru Kimura. Toku fans will probably know him best as the voice of the Seiza Blaster in Kyuranger. However, since then Kimura has made numerous social media posts mocking African cultures and on one occasion even posted a photo of himself in blackface. These posts were removed very quickly after getting many negative responses, but he has yet to comment on them in any way and blocks anyone that brings it up.
Kimura is half German (his real surname is "Bartsch") and was born in Germany. He is also fluent in English and lived in the US for a time when appeared on Broadway. So the usual "it's a different culture" argument doesn't apply here.
Anyway, stepping away from that horribleness...
The head writer will be tokusatsu newcomer Hanta Kinoshita.
The main director will be Takayuki Shibasaki. I believe this is his first time as head director (?), but he has directed episodes of the show for pretty much every season since Kabuto.
The series is produced by Chihiro Inoue (also a main producer on Kyuranger), Kei Mizutani (who has produced a few episodes of Saber and has the same name as the actress who played Shelinda in Gingaman, though I don't know if they are the same person?), and Taku Mochizuki (who worked on Kyuranger and Kiramager).
The last thing to mention is that Revi and Vice have already debuted! The film Superhero Senki has just been released (July 22nd). In the movie there is a location called "Agastia Base" where the boundaries between fact and fiction are blurred. This place stores books containing the stories of all the Kamen Riders previous to Saber, and Revice's book is shown along with them. Then after the main film ends there is a 23 minute long film dedicated to Revice.
The official press conference will be held this coming Tuesday, July 27th. The series will begin on Sunday September 5th.