Pretty much follow the same guides for the single player achievements as the MP ones.
The only difference is the amount for gold on any mission that isnt a race. Race times do not change, you only need both players to cross the finish line. Having said that, there is something important to note on races, both players MUST be on the finishing flag, not past it. Particularly on the cycle races and the car race in the tunnels, if the flag is not turning orange you're too far away and have to come back. If your teammate is not near the orange flag, the race will not end.
Having said that, there are a few changes worth noting between the single player and co op strategies.
Mission: (The co op version of Thirst Quencher)
Requirements: Drink 60 booze in 3 minutes
Don't try to dump all the booze on one player who's holding the drinking magazine, it wont work, you'll literally run out of time due to the player drinking animation and picking up new items.
You also cannot duplicate the drinking magazine, have one or both of your players wear the sports fan outfit, this will speed your running rate up and make you immune to alcohol induced vomiting.
Mission: Picture Perfect Partners
Requirement for Gold: 11000PP from Photos
There is very few ways of getting PP effectively in this. Because of the double-exposure issue with photos, two players cannot run the same single player tactic of using the brutality shot from the alien UFO crash ride.
The best way to get the PP easily is to ensure you have the Erotica Magazine from the Slot Ranch Casino's mens room (Doubles PP from Erotic photos) and the two photography magazines. Give these to one of the two players and have him photograph the anal probes on the path to the alien crash ride. The first photo should net around 6000PP for a close up shot of all of them, second one will net 3000, then 1500, etc. etc...
In 60 seconds, this will be hard to ensure that you get all the XP since hte photo cooldown takes time, but two photos plus ones like the BFF shot, or the space probe brutality should push you over the 11,000 mark.
Mission: (Whatever it's called), the Co op version of Extreme Off Road
Requirements: 800 Kills
Even with the slice cycle and the SUV, you will find yourself running out of zombies very very fast. The best method to get your 800 kills is to have both players use a pair of Super BFGs each, these will easily street sweep the area and ensure that you dont miss any zombies.
Using the vehicles, even the slice cycles and SUV will end up thinning out the zombie numbers too much to be effective and you'll be lucky to hit 800 before the end of the time limit.
Mission: Lightning Strikes Twice
Requirements: 300 Kills
Don't even bother with the electric crushers, you'll never be able to get enough kills in time. Have you and your partner buy a pair of tesla balls from the pawn shop, stack them in the maintainance room behind the challenge star, then buy two more. The 4 tesla balls will easily net you close to 400 electric kills before the time limit runs out.
Mission: (The co op version of Mad Hats)
Requirements: 30 hats on Zombies at once
Be quick with this one, and dont stop planting hats on zombies even after you hit your 30, the hats WILL FALL OFF and you need to keep up the hat stacking right up until the last minute. The rate that hats fall off is random, so you'd need to keep going to ward off the slipping under gold that can occur if you dont focus.
Mission: Twin Terrors
Requirement: Kill one twin in 50 seconds
Rather then using the laser sword, have both players use the drillers instead, quickstep and driller, pick a twin and focus on her, your and your buddy will be able to keep them from escaping because of hte rapid attacks and when you both gang up on the one twin, you do as much damage as the laser sword but at a much faster rate. On average you should kill one twin in under 30 seconds.