Some think that Co-Op takes place after you beat singleplayer. Personally though, I think that it takes place before Chell arrives. Here is my reasons.
1. Right before you are launched into Wheatley's first death trap (the one where "The Part Where He Kills You starts.), GLaDOS says that she built the robots before Chell arrived.
2. You see P-Body during singleplayer as an easter egg
3. Atlas and P-Body are shown in the boots video, although wouldn't that mean Cave Johnson built them?

I'm pretty sure it takes place after the single-player campaign.

||1) she was building them, not testing them.
2) That was Wheatley testing P-body, not GLaDOS. He'd already found them in storage by that point and was secretly testing them to avoid Chell and GLaDOS knowing that they weren't needed anymore.
3)All the technology seen in those videos is modern stuff, Cave Johnson is just used as a narrator. I don't think it's canon.

4) GLaDOS seemed not to know about the underground areas in the singleplayer campaign, and in the co-op campaign she sends the bots down there to get all the humans.Had this occurred prior to the singleplayer, GLaDOS would be aware of what was down there and not confused about who Cave Johnson/Caroline was and so on.

5)Also, the bots are seen at the end as GLaDOS gives her ending speech. This seems like a sign that she'll test them instead after you leave, after all, you were her only remaining living test subject.||

I'm pretty sure it goes SP and then Coop.

ITT:

Hober wrote:
ITT:

I found out that ITT meant I Think This on the first try. Do I get a cookie?

Godzilla (Lazars) wrote:
Hober wrote:

ITT:

I found out that ITT meant I Think This on the first try. Do I get a cookie?

I was under the impression it meant--in this situation, at least--"In This Thread:".

WinstonSmith wrote:
Godzilla (Lazars) wrote:

Hober wrote:

ITT:

I found out that ITT meant I Think This on the first try. Do I get a cookie?

I was under the impression it meant--in this situation, at least--"In This Thread:".

NUUUUU

WinstonSmith wrote:
"In This Thread:"

ITT COMEDIANS TELL VIDYA JOKES

Hober wrote:
WinstonSmith wrote:

"In This Thread:"

ITT COMEDIANS TELL VIDYA JOKES

That's right, I know my Internet acronyms.

GLaDOS mentions the events of the single player campaign to the bots in the lobby. It takes place afterwards.