alright here's what new for us with black list it's about sam partner for co-op notthat differnt from what happened with Rainbow six vegas (the only thin that's the same about these two is you have three to select from there's solo and co-op for story mode and you have the specialized co-op multiplayer)
Co-op stealth and survival in Splinter Cell: Blacklist

(5 hours ago)
9

Despite being the boss of his own intelligence team, Fourth Echelon, Sam Fisher is still keen to
go it alone on most missions in
Splinter Cell: Blacklist. For those
special outings, he'll need a partner.
Splinter Cell: Blacklist's co-op consists of narrative missions, pulled directly from Sam's subordinates on board the Paladin – a massive plane and base of operations for Fourth Echelon in
Splinter Cell: Blacklist. Missions will run the gamut from stealthy to more action-focused affairs and in-between, forcing coordination and cooperation in order to reach the next objective.
Or, in my case, simply survive.
Splinter Cell: Blacklist (5/7/13)
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and here's details on spies vs mercs
Spies vs. Mercs evolved in Splinter Cell: Blacklist

(6 hours ago)
17
Splinter Cell: Blacklist's Spies vs Mercs mode offers a sense of claustrophobic fear that few multiplayer games have been able to offer. It's an asymmetrical assault mode where one team of heavily-armed mercenaries attempts to defend three terminals from nimble spies attempting to hack in and steal the ones and zeros within.
No matter which side you're on, when one terminal is being hacked, all of the conflict and chaos is focused like a laser to one room where the mercenaries must hunt for the original hacker to stop the countdown. You can't leave that room until the job is done – either the terminal is hacked or the hacker has been dealt with. It's about the most tense 90 seconds I've been asked to endure in a video game.
source:JoyStiq