Dead Space 3

There are 3 missions that you need your coop partner for.
The first is in Chapter 4 and is called CSM Brusilov.
The second is in Chapter 11 and is called Archaeology
The third is in Chapter 14 and is called Marker Containment
TIP: In Marker Containment, it may seem like the enemies are endless and you will be fighting forever, bit in the room where Carver gets sent, there will be a marker. You need to shoot and destroy that marker, and you will be sent back to normal land. In the final room, the marker will be on both Carver's and Isaac's side. Destroy them both.

At any point you may go to Chapter Select and boot up the level right at the start of the coop mission. Simply invite your partner in and get killing! Get to the end of the mission, and as soon as it says it is complete, save and quit, then boot up the next coop mission, re-invite your partner, and rinse and repeat.

ALSO! Do not forget to get those collectibles in the coop missions. It would be best to not have to go back through just because you missed one collectible.

This might seem similar to Sargeant Renkor's "guide" but I've got some things to clarify and other things to say.

Like Sargeant Renkor said:
"There are 3 missions that you need your coop partner for.
The first is in Chapter 4 and is called CSM Brusilov.
The second is in Chapter 11 and is called Archaeology
The third is in Chapter 14 and is called Marker Containment"

In the Marker Containment Side mission there are 3 parts where Carver fights inside of his own mind while Issac is ambushed by black hardened necromorphs (and yes they are endless until Carver completes the event in his mind). You may be having trouble while carver and here is some good advice. At the beginning of each event (as Carver) don't stop running. run around (to the right side) all the shadow minions, heal up if you get hurt (it does use a health pack in your inventory) and towards the couch where the marker and your wife are. The first time you get to the couch. The event will end.
NOTE: The enemies in Carvers mind are also endless. Hence why you should just run around.

The second event will occur and Carver's wife will be sitting on the couch. This particular event seems like it's timed. IMPORTANT: ******Go as close to the couch as u possibly can and the shadow minions will just stand still and wont attack you.******** If you seem to be waiting a long time (as I've not yet verified if this event is timed or not) then there should be a marker basically touching you on the left of the couch, shoot that until the event ends.

Now the third is the most confusing for people because at no point does it ever indicate that you should shoot the marker and the shadow minions do a lot of damage and of course are endless. What you must do is shoot the marker until it explodes.The same spot that was mentioned in event 2 also works for event 3. Stand in between the couch and the marker and you'll be safe from the shadow minions while being able to just point back trigger mash that damn marker.

***THINGS ABOUT THE MISSIONS THAT YOU MIGHT NOT KNOW***: If you play through the entire mission then you get little extras that you wouldn't get if you just jumped straight into the mission from the menu.

1st - and most obvious, you get the extra dialogue exchanged from Issac and Carver.

2nd - There is actually an extra cutscene that introduces the 3rd Co-Op mission to you.

3rd - Some things that carver sees in the missions are different then what Issac sees. (Same thing goes for Issac seeing different things than Carver)

4th - There are audio logs in the Co-Op missions that have different messages depending on what character you're playing. (Ex. As Issac you hear a man laughing over and over and over again. As Carver you hear a message from his wife.) Things like that.