Are you really questioning whether it's against the rules to act on information that your character doesn't have?
Yes.
To give the simplest example (which I think [MENTION=16586]Campbell[/MENTION] used upthread): if I as a player know that such-and-such a character is the new PC for another player, then it is not against the rules, and is probably good for the game, that I have regard to that in deciding how my PC reacts to that newly-encountered character, even though my PC does not have any information that marks this newly-encountered person as having any special status or significance.
To go from particular example to general principle: where is this rule stated in (say) the AD&D 1st ed rulebooks? It is stated in Molvay Basic, but then Moldvay Basic also says that the rules are mere guidelines to be adjusted/adapted to the needs of play - which is exactly what Campbell is suggesting in the OP.
The closest thing I found on a quick skim of the 4e PHB was this (on p 8):
When you play your D&D character, you put yourself into your character’s shoes and make decisions as if you were that character. You decide which door your character opens next. You decide whether to attack a monster, to negotiate with a villain, or to attempt a dangerous quest. You can make these decisions based on your character’s personality, motivations, and goals, and you can even speak or act in character if you like.
I have bolded the word
can, because it is not synonymous with
must. It implies the possibility of doing otherwise, ie of relying upon motivations and goals that come from other than the PC (eg metagame ones).
All I could find in a quick skim of the 5e Basic PDF was the following (p 2):
[T]he players decide what they want their adventurers to do.
There are no limits stated on the motivations and reasoning that the players may deploy in reaching such decisions.
I'm having difficulty parsing your example here
One reason for this is that I have a negation I shouldn't have! - I've edited to remove it.
But restating the example with a bit less compression:
I'm imagining that PC 2 (played by player B) has a dark secret, which PC 1 (played by player A) doesn't know about. And I'm further imagining that the GM has framed the PCs (and thereby the players) into a situation in which PC 2's dark secret becomes salient.
In a situation like that, player A has (at least) two options: (i) s/he can play his/her PC in accordance with the PC's knowledge, which includes ignorance of the dark secret; or (ii) s/he can make choices for his/her PC which will help drive the game forward in relation to the dark secret. Here is an example of (ii): player A can decide that something about the situation the GM has described catches PC 1's attention, even though the only reason for deciding that this is true of PC 1 is because player A knows that this will help player B get more deeply engaged, in play, with PC 2's dark secret.
I think that player A taking option (ii) rather than option (i) is almost always a good thing, despite being metagaming.