You have to do a lot of thinking about this, and I mean a lot.
Players are smart little bastards. They'll design traps and lie in ambush for the other guys. People will get to hiring NPCs and winning over townsfolk to fight against the other guys. They'll get into a tussle and it's liable that they end up killing one another before any real 'elder evil showdown'.
There are a number of ways to get around them running into one another prematurely, but none are particularly good: if you toss down a godly hand and flat out prevent one group from trying to get a jump on the other without explicit reason other than 'you all shouldn't fight yet', you're squelching player choice. If you put them far apart on either side of the continent, it might work for a while until someone learns to teleport and discovers the location of the other PCs. Letting them duke it out every now and then might be best as to give the illusion of choice and control over the situation for the players, but you're going to want to step in at some point to prevent a TPK one way or the other. How I wouldn't know, but what ever you do do not toss in monsters. A cast of Dominate Person can mean life or death when a new guy pops onto the scene, especially if he comes in mid-fight when spells have been used up and people are already beaten on.
This is also an overwhelming job, taking on two parties. Personally, one adventurer is enough for me, let alone four or five. When you double that, you're keeping track of a whole lot more. If you're really aiming to do this, then you have my support in the matter, but please keep in mind it will not be an easy task in the least.