Rogue Coop 101?

[Zenda]

But first displaced Monsters... and now the partners are showing up all willy-nilly on different spots for each player?

Normally, a player character's position is pretty well synchronised. Only when someone teleports or moves into a location that's occupied by a monster on other screens, their position may be off a tile, and it's immediately corrected when they take another step. A mismatch in monster positions is much more common, but it can be countered as Nystul described: use shift to shoot along walls, and don't hold your cursor over a monster when doing so.

Let me try to explain how that works. If you shoot while pointing directly at a monster (even while using shift), that action will be communicated as a shot from your position towards the monster's position. If that monster happens to be somewhere else for others (moving or not), those others will see your arrows fly to their location of that monster, which might well be in a different direction. So, since you don't really know where your arrows fly on their screen, you might be hitting someone you don't want to hit, without knowing. However, if you shoot without having a monster at your cursor (using shift), the shot will be communicated as going towards a specific dungeon location, given in coordinates. This way, the arrows will fly the same for anyone in the game. This will also make it easier for your coop partner to stay out of your line of fire, since they will now be able to tell where your arrows will go, just by knowing where you are. When all players in a game understand this mechanism, it's not uncommon to finish games without any friendly fire at all.

For example, going by this knowledge, for my coop partners and me a typical setup against non-overwhelming odds is something like the following:

/ wall
+ tile
M Monster
W Warrior
R Rogue
x cursor position

///////////////////////////////////
MMM++++++++W+++++++++++
MM+++++++++++++++xR+++++ <- best Rogue spot against crowds
M+++++++++++++++++++++++
+++++++++++++x++++++++++
++++++++++++++R+++++++++ <- best Rogue spot against single monsters
++++++++++++++++++++++++

Another great benefit of this 'non-aiming' fire is that works very well with knockback on your bow: since you're not targetting, there is no need to re-aquire a target.

How DO you coordinate?

By knowing what the other(s) will do. Once you make a habit of using a setup like above, there is no longer much need for communication. The warrior knows he must get to the wall and stay there, the rogue knows where to stand and how to shoot. We only tend to discuss tactics in-game for dangerous (boss-) packs, when we know in advance about their whereabouts. Even then, it usually comes down to picking a spot and type of setup to use (the one above is good against melee, but not so against ranged attackers, for example).