Left 4 Dead 2

This cannot be done on your own. You will need two other players to activate the other generators during the finale for this to unlock for you.

All three people have to be at a generator at the same time. Communication is key. Count down and all of you start the generator at the same time. It should pop up right away. If not you had a timing issue which has happened to me.

So grab a couple friends and get at it.

Here's a nice video from Roosterteeth that show how to do this. Just make sure you are playing with at least 2 other human controlled players. Make sure the three humans are next to the generators. Start a countdown for when everyone should start their generator. Achievement should then unlock!

If you have the luxury of three controllers this is still not attainable solo as l4d2 only supports 2 players splitscreen.

You will need to do this over system link or online and the latter can be troublesome. Three players work as do two players where one player has 2 controls with a guest or secondary account on one of them. So getting this cheevo is all well and good with at least one player with a mic so that they can synchronise the generator start with a verbal countdown but what if both/all parties are without a microphone?

Here's how I realised you can work past it; simply set difficulty to easy and start killing zombies. When it's time for you to part ways to each go to your own generator incap the fourth effectively useless player [call a vote to change difficulty to expert only shoot the player enough to incapacitate them then call a vote to change difficulty back to easy]. Now each of the three players must go to a generator and aim at the switch in such a way that if you tap x it should begin to start the generator but instead it says wait for all players to be ready. The moment the incapped player dies [a bloody cross will strike their name on the hud] everyone quickly holds down x until all the generators are started and achievements are unlocked left right and centre! :D Seeing as having a player incapped will not allow you to start the generator until they are dead this method of synchronisation is even easier than you would expect! Additionally, the incapped player's hitpoint/health bar slowly decreasing is effectively like a visual countdown in itself.

Hope this helps
Happy gaming!

This achievement is at the beginning of the finale. You will need 3 people to start the generators at the same time. Make sure you do a countdown with your fellow players so it is timed right. The achievement will pop up immediately, so if it doesn't vote back to lobby and retry.

The best way to do this is to hook up a 2nd controller and position your 2 characters at a generator each(that way you only have to worry about 1 other person timing this correctly). Then have a friend hit his generator at the same time and the achievement will pop up.