See here.
https://itch.io/docs/creators/quality-guidelines#avoid-obtrusive-advertisements-...
Self promotion is not what is discouraged there. Self promotion is basically expected from indie devs, I am not even sure, if it can be called ads. But advertisment in games nowadays often is associated with data grabbing, adware and other kinds of dubious things. Especially on mobile platforms.
If you do have server connections for your self promotion though, that might give some backlash. Someone with a firewall might complain and give bad reviews.
As for visibility, my gut tells me, that releasing them one after the other would increase visibility. But if you do link to the second game inside the first game, while the second game is not accessible, that might lead to confusion.