[...] if one player has entered into a dialogue, then his friend can only observe and does not affect the dialogue in any way / does not add phrases of his class. And this is very sad, because, for example, I play with a friend, I am a diplomat in the team, but he is, say, a wizard - and when it comes to magic in conversation, I would like him not just to stand in the background, but to intervene in the dialogue and be able to say his unique phrase of a wizard.
Is something like this planned, perhaps someone knows? [...]
As Wormerine said, this was requested multiple times. But Larian's EA communication policy is to give no information on what will be updated and what will not ... and in fact not even on what feedback has been recorded/heard. So, no, we don't know.
If you want to enter the realm of speculation, well ... on the one hand, it sort of seems that Larian is designing the game with the multiplayer gameplay as their priority, so one could hope they would fix this. (It's also a very basic, very natural, very DnD, very "gather your party" feature). On the other hand, from what I've heard, D:OS2 didn't have this feature. And it's not there in BG3 so far, which Larian has been developing for years now. So, I think your bet will be as good as any.
It's very unclear how much Larian actually tries to collect the feedback given here, on their official forum. If you can spare some time, find the feedback report form in the Larian launcher. With some luck, they'll read it.
And the more people request this, the more likely it is that Larian will decide to implement this, if it wasn't part of their plans. (And by more likely I mean not strictly less likely. It may change nothing, but it won't hurt.)
