Me and my buddy Mason have been learning Halo CE co op easy speedrun and we found that people A: don't really run it and B: the people that do just use solo strats. With the two of us its much easier to stack many grenades in areas to potentially break things. Also we can abuse loading each other in hallways for some missions to save time. This jump is possible solo but we modified it for our co op speedrun as the solo skip to the 3rd floor is insanely hard. Let me know what you guys think!
You can also have one partner have full grenades and backsmack/assassinate them while they are looking down and it groups the grenades pretty well most of the time, then throw one grenade as the surviving party and pop that way. It takes practice to learn where you need to do it, but it can be real quick without requiring a lot of communicative timing.
I highly suggest muting the video as I swear right off the bat on this link, and I'm really annoying as this was a year and a half ago recorded. So, it was not practiced at the time and if you rewind you can see how bad I am at the timing.
But with practice you can get pretty good in co-op to where it takes literally about .75 seconds of set up.