Is it necessary to be in the same server for co-op game play?

no...unless your a noob..lol

Dark souls, unlike the vorige Demon Souls and upcoming Dark Souls II, uses a Peer2Peer based system for PvP matchmaking purposes.
Explained simply this means that u ARE the server as opposed to connecting to one.
To attempt to further explain, when u load a save file u put your name into a hat and over time u will randomly pick names out of this hat. This hat is roughly based on region and level and can be quite large AND only consists of people online right then. These people are those who appear as phantoms, oranje soapstone signs, summons, and even invaders. Any interaction with players is only from players in this pool that u build and this can create difficulty with both PvP and Jolly Cooperation in a few ways.
Because one of u is host, there WILL be lag and this can lead to Many annoying things such as not being able to parry, dodged attacks hitting, and PvP back stabs too much. Additionally this can impede co-op play because although u have 'picked' your vrienden character they may have not 'picked' yours leading to summons being tricky and prone to failing until they finally work where it will force u to be added to their 'pool' of players. I word that as such because if they were not in your pool u could not even see their sign. Besides that issue summoning is iffy anyways.

A word of advice/warning: quitting to the titel screen will clear this pool so trying to summon immediatly after aan het uploaden will be difficult as it takes some time to add players to your 'pool' again.