Looks like fun.
Alan is role-playing #2, while acting #3. Bob is portraying the NPC through acting, he is not role-playing as defined under #2. (this is because DMPCs don't work, you can't objectively test yourself as a player in a game you're refereeing (any kind of refereed game)). Bob has rules for how his NPC can behave (act #3). Alan, the player, doesn't.
(think bluff, sense motive, attitude adjustment, morale, etc.)
Same as above with two roles acted.
Same as above, except now Bob the GM is simulating the environment watched even though he is acting out characters in that environment.
Alan is role-playing 2 PCs. He has limited control over one.
EDIT: Bob is just acting his one NPC, but see below too. Forgot about his NPC here.
In some games Alan's control over the the activities of the Cohort would cover any obeyed orders when given from Alan's actual PC (the Batman to his Robin). The DM would step in when required.
In other games, the limit on control would be less or even gone. The cohort could be his full PC.
Alan's #3 role-playing (acting) of his main PC as "forgetful" would be a result of Alan's intention of role-playing to act #3 (One of many intentions possible while role-playing in a simulation #2). That's his prerogative. As they say, "you can't tell someone how to role-play their character in an RPG". Type #2 role-playing does not have rules for how to portray your character.
Alan playing his Cohort stepping in to remind his main PC of forgotten details means Alan can (depending on the degree of assistance he gives "himself" here) act #3 to limit his successful role-play #2 in the world, while still playing towards success overall. As Alan gets to play 2 PCs (1 limited or not) to succeed as a team in the world, he can limit one when in discussion with each other and not hurt his actual chances of his success. Playing the knowledge of two PCs, while only one player limits what they could do if run separately (2 player's abilities tested).
In my other reading of your question, where the DM is stepping in as the cohort to help Alan (whether his forgetting is feigned or not). Not Alan as the full PC cohort:
This is like any NPC interacting with a PC may or may not be assisting that PC. The question is: Is the DM giving an objective portrayal? Is he only using knowledge the NPC has? Behaving as the generated character?
If not, the NPC may unfairly be played to assist Players who need help and this is the GM "bending the rules" to make their game play easier. Just as if he were fudging the dice in the players' favor. He's fudging the simulation in your favor. This is your benevolent tyrant.
OTOH, the NPC may unfairly be played to hinder the Players who are doing something the DM doesn't like and is "bending the ruls" to make their game play harder. He's fudging against you, portrayed NPC or dice roll. This is your "Killer DM" tyrant.
Either of these are called in the Big Model:
Illusionism - the GM is warping the world without players knowing,
Participationalism - the GM is permitted permitted by the Players to warp the world.
I believe objective portrayal of the simulated reality is best. That's my preference though as it doesn't end up in "we won because the DM let us win". This is "let us win or lose" just as in a court case where we are sentenced by a judge breaking the law. Who really wants that?