2 boxing is allowed. 3 boxing is a no no. My personal opinion about 2 boxing is that it should be avoided. I like to focus on 1 character at a time and not have to bother alt+tab or keeping track of the bot. Some people do however enjoy 2 characters. It does have the benefit of always having a "friend" to group with. The downside of too many people 2 boxing is the lag to the server. Performed properly on a good pc or 2 good pc's is less stress on the connection, but most people only have 1 pc and so 2 box on the same pc. This is perfectly ok, but please keep in mind that even a smidgeon of lag can contribute to a tidal wave of it.
Which is why I never 2 box. I don't want to contribute to any kind of lag. Even though per computer/connection, it might be a very insignificant amount contributed to a zone's lag, concider 300 people 2 boxing in the same zone then....now it's not a tiny amount anymore.
If you really want to 2 box and try to do it on the same pc, maybe try having 2 hard drives and a 2 cpu mother board. 2 or more gigs of ram and a 256 graphics card. Also having 2 monitors would be a huge plus. I have a pc with 2 NIC cards plus wifi (no dial up modem). Each eqw could be on its own drive with its own resources, own monitor and possibly no lag at all. Assuming you have the money to buy and/or build a monster game pc like I did. And also assuming you have a good bandwidth(internet connection speed). My bandwidth is 54mb, so I zone fast and I get very little to no lag.
You might want to concider upping the virtual ram/pagefile also for effective 2 boxing. The post about having 2 directories, that's a good idea. Technically, you dont have to install EQ twice, just copy/paste the **** folder somewhere else on the drive, or in my case, to other hard drive. If I was to 2 box, I would have 1 directory on each drive, each with its own dedicated resources. 4 or more gb of ram should be plenty for 2 boxing. Technically you can do it with 512 mb ram. I've seen it done on a dinosaur laptop.
I guess personal preferences are going to determine if you 2 box and how you do it. But just bear in mind that to do it right and not lag a zone to poo, try to do it on a decent pc. a little bit of planning and system resource management will go a long way. On lower end pc's, just go to control panel, system and up your pagefile. If you have the Hd space, make it 2 gb or so. It will make a difference when you 2 box.