The Infinity series loves this trope. In short, if two characters share a voice actor or one character has their voice actor hidden, you can bet it's going to be important.
Never7: Haruka and Kurumi, because the former is a clone of the latter.
Ever17: Takeshi and the Kid. Slightly complicated- on one hand, this is because the "Takeshi" and "Kid" that are physically seen by the other character turn out to be the same person. On the other hand, the real Takeshi and the second Kid have the same voice actor, but aside from being father and son, they have no reason to share a voice actor. Furthermore, in the Drama CD "2035", Blick Winkel shares the same voice actor as those three. The two Yous also have the same voice actor.
Remember11: Yuni shares a voice with Junichi for no real reason. However, when Yuni manages to calm Utsumi down from her Unstoppable Rage against Hotori, the fact that his voice is the same as her deceased son's contributes to her finally calming down.
Also occurs literally in Bad Ending #28, where 2012!Satoru ends up in 2011, and meets 2011!Satoru. Notably, while 2012!Satoru has a standard "young man" voice, 2011!Satoru has a cold, deep voice.
12Riven: Shinkuro shares a voice with Ohtemachi, Maina shares a voice with Narumi, and Omega shares a voice with Renmaru. The first one is because the former is an RSD program created in the likeness of the latter, in the second, the former is the physical manifestation of a part of the latter's mind that she lost (It Makes Sense in Context), and the third one is because the two are the same person.
I/O: Twins Mayumi and Masami Shinozuka have the same voice actress. He shares a voice with Hinata, "Ashur", and Isaiah, and Lem shares a voice with Mutsuki, "Marduk", and Ereshkigal. A bunch of other characters share voice actors, but this is because the voice cast is about half the size of the cast.
In Umineko: When They Cry this happens between Eva and her Enemy Without EVA-Beatrice in Episode 3. And also with the "Chick Beatrice" and the Elder Beatrice in Episode 6. Outside of that, Sayaka Ohara voices many versions of Beatrice throughout the series.
An interesting case happens in the english dub of Dangan Ronpa where Toko Fukawa and her Split Personality, Genocide Jack (or Jill) has two voice actresses who plays them (Amanda CÚline Miller and Erin Fitzgerald respectively) they are also known to play two halves of the same character with multiple split personalities; Junko Enoshima.
In Sunrider Liberation Day, Chigara shares a voice actress with Lynn, Alice, and Alpha. Justified, as all of these characters are Prototype clones.