Viper and Daemon made posts about this in an older thread:
viewtopic.php?f=6&t=29Nostalrius is wording this differently, but from what I gather, yes multiboxing will be allowed as long as you aren't using any third party programs to control, automate, or bot the characters. Alt tabbing, which is control of only 1 character at once, does not fall under this. This means the rules here will be similar to Blizzard's, although Blizzard didn't actually enforce theirs. We were left to assume that Blizzard liked the extra sub fees, since they allowed almost everything that violated their ToS. Or they'd just slap players on the wrist, who would promptly go right back to doing it. Whether Nostalrius will enforce its rules or not here remains to be seen. But players will inevitable come along and do things like this, and you'll see if they are kicked out or allowed to remain. Hackers will also come along, and its inevitable that they will find a way to bypass whatever prevention measures were meant to keep them out. You just have to see if they are dealt with, or allowed to remain on the server.
But even if you multi box legitimately, you can still violate other rules, such as PvP exploits. This is something to remember. It is not ok to farm yourself for free honor points or rig BG numbers so your side has an unfair advantage(ie 15vs14 or worse). On most private servers, you can very easily rig BGs in your favor using a second character. Small populations means the BG pops for both characters and ends up in the same match. You then alt tab often enough to keep the other guy in the match, doing nothing, so your primary character gets to fight on a bigger team. Assuming you want to farm up both, then they both get the benefit as you just trade which you play vs which you AFK. BGs are very easily determined by team sizes. Your win rate would sky rocket thanks to skewing every match to 10vs9 or 15vs14. These are problems that plague other private servers thanks to poor regulation from staffs that don't care about them.