The other player was Iron Monkey.
Having four characters with their own agendas did pose some challenge to putting sufficient focus on each. The two overwhelming methods I made use of were some immense external threats that required response from all the characters and, from time to time, simply saying "you and you are here and this is happening", which I believe is explicitly set out in the MCing section of the book as an MC prerogative.
There were a few more indirect pressures that motivated the players to involve their characters with each other. One, the Hx rules (primarily the 'rollover' xp for 'who knows your character better') got them trying to get in one anothers' business. Two, several of the character moves spell out NPC involvement, and those NPCs have relationships (of some kind) with the other characters (this is a basic implementation of the 'PC-NPC-PC triangle' discussed in the book). Three, the other players' characters matter, frankly, and are doing interesting stuff, by their nature as protagonists; this is backed up by their being way tougher and more effective than anyone else, when they stay proactive.
Whoops. I re-read what you asked - I didn't do much to enforce separation of character and player knowledge. I am lucky to be playing with people who are either fairly entertaining or easily entertained, and so they usually remained interested in one anothers' scenes when separate, and also ignored or made use of out-of-character knowledge as appropriate to the table's enjoyment.
Now that I think of it, there was a bit of note passing, when Iron Monkey first began to draw antagonism from the other players, but I didn't encourage it - I felt the game handled both subtle and open antagonism between the PCs just fine, as did the people I was playing with.
Hope that helps! Any questions are welcome.