Since increasing corruption is part of the core mechanic of the system and is resetting your echo web, it is not that simple to just increase corruption for both players.
There are also potential abuse cases and RMT things to consider.
If one player could prepare a high corruption gain shade and the other player would benefit from it without the needed work and preparation in their own echo web, that will lead to a lot of abuse cases.
Ok. I can understand that. But that kinda seems like amputating an arm for a hang nail.
The solution to fix fears of someone being boosted is to make all group play frustrating and inefficient?
Right now it really seems that solo play is so much more effective than group play as to make group play feel like punishment.
Being forced in to, “this sucks,… let’s just play by ourselves until late endgame because it isn’t fun advancing so much slower” doesn’t seem like an effective solution to fix the problem or fear you are describing.
That is really not true. It might feel like that because of certain things, but there are a lot of things that are really really advantageous in group play.
Monolith rewards, Rune of Ascendance, double the boss drops etc.
I feel like this works fine as is. Just like maps in PoE. I’ve always played with a friend in PoE and we just took turns advancing the other one’s maps. Usually one has a stronger character and is boosting the other.
There are already advantages to playing in a group, like @Heavy mentioned, plus it’s faster to clear/get XPs overall.
EDIT: not to mention that if both raised corruption you could easily end up at a corruption level that one of the players couldn’t handle, and that would leave that player feeling bad as well.