Note: I achieved this on splitscreen, but without peeking, nevertheless some modifications should be applied when playing through xbox live.
In the beginning some general guidelines:
1. As mentioned above - communication is the key to success, at all times you should know the position of your partner.
2. When undetected (which is your preferable state), try to go seperate ways, but remember not to be too far from your teammate. Otherwise when you go down, the help won't reach you in time.
3. Weapons - I used Five-Seven and MP5-SD3, both fully upgraded. First one because of 4 marks, second one because of combination of silenced gun and scope. Most of the kills were made with handgun, and MP5-SD3 was used when necessary to kill on a long range without being detected.
4. Gadgets - try to avoid using them, because you get detected easily. The handiest for me were remote mine and sticky camera (when thrown they go unnoticed until explosion). As to portable emp - very very useful when too many enemies are around and you are in a bad position; use it, run to a better place and take 'em out!
5. There are two big differences between realistic and other difficulty levels - enemies are better at aiming and you get easily detected (sometimes impossible to avoid). Remember about it at all times!
Now about problematic moments in the campaign:
Scene 1, Chapter 4:
Last part of this chapter may be problematic, because you are sorrounded by cops you can't shoot, but you can knock them out (B). They appear when you try to leave the room after Lesovsky's phone call. Stay there, wait for the one cop that will be patrolling the building, knock him out, afterwards go to the right exit and when you are ready, use portable emp (it had to be used twice in our case) and run along the right wall, there you will find the door to the corridor.
Scene 2, Chapter 3:
The very ending (when you throw the IR strobes and wait for the chopper) may be problematic. The key is to hide in good places. In our case it were the two ventilators on your right (when looking at the building), the person in the back takes care of stairs on the opposite border. Don't try to escape at once, when one of you jumps on the chopper, the second one still shoots.
Scene 3, Chapter 2:
Just one tip on hacking the computers in library - if you want to make it in time, you have to take care of two computers at once, so each one of you has to take care of himself.
Scene 3, Chapter 4:
This took us a lot of time. We found it much harder on realistic difficulty and needed a bit of luck ;) The general tip is to try to remain undetected as long as possible (though at some point you will be detected). Shoot the lights (the ones on the roof), take down the patrolling guards one by one. One of you has to reach the opposite wall of the room to flip the lever, while the other person is waiting near the entrance (the best place is the part of the pipe in shadows), where the second lever is located. The person that goes to the other side: go as much to the back of the room as possible, remember that you still will be detected by the turret and the guards will go after you. You are on your own. The second problem - when you want to pull the lever, the turret will detect you again, and this is the moment when you need luck, because you will have problems with controlling your moves (at least I had). If you survive and the other person will pull his lever (with same problems), you just shoot at the crane and this is it.
Scene 4, Chapter 4:
The last scene (with Antonov) is the toughest. I mean really. A lot of swearing and wordfights are unavoidable ;) We had a tactic that worked well on easier difficulty - when you want to reach the room on upper level of the building located in the far right, you just run together and release the EMP two times on your way. the same thing applies when you are going back to the plane. Don't even bother to shoot enemies, because it takes time. Just take out those in the room and nearby. On realistic difficulty we had to modify it a bit. At the beginning kill as much enemies as possible. When you avoid detection, more will not come. With every detection three more enemies get to the hangar. After getting rid of most (or all) enemies, get to the room, turn the lights off (because the turrett will see you) and release the pulse. Afterwards more enemies will come. the best way is to get to the ground floor of this building (inner stairs), take the lights out and avoid being detected. Wait there until you are sure they don't know where you are and then... Either be sure that they don't see you when you go through the light, or shoot the explosing can to shift their attention. In this moment... run! And release the EMP's as many times as possible. This worked, but after many attempts...
If you have any other questions please write!
Thanks for reading (hope anyone got through it :D)