Is MultiBoxing Against Terms?
Is it against the terms of service to MultiBox this game? If not is anyone out there doing this?
For those how don't know Multiboxing is a term used to denote one user playing multiple accounts simultaneously. This can be done using one or more machines.
While there is nothing in the terms that appears to directly confirm that you can't multibox, the terms do ask that you use common sense and keep with the spirit of the game. If you multibox using a single machine, it is likely OOO will be able to find out, and you may end up being punished regardless. Remember that most MMO companies include a clause in their terms and conditions that allows them to close your account for any reason they deem fair, and they may decide you're setting a precedent for a new rule in their terms.
I say this because it's easy to see how multiboxing could be abused, so it's also possible it may be punished.

Based on how OOO has treated Yohoho! Puzzle Pirates players, you use as many paid accounts as you want. by "paid accoutn", I mean you have spent real money buying CE from OOO, not just CE with crowns. OOO limits you to creating 3 free accounts per computer per some period of time, I think around 90 days, and over the years I've made about a dozen YPP accounts.. You can launch multiple clients on the same computer if you want, but of course that will cause them to all share ME.
The need for multiple accounts, especially unpaid accounts, is much less in SK than YPP. If you are using this feature to do bad things, expect to get banned, but that is for doing the bad things, not for using multiple accounts.
I have heard nothing that contradicts my experience with YPP, but I anyone knows of official statements to the contrary, I would love to learn about them.

What would you count as "doing bad things" then?
Carrying an additional character you own through a gate to earn double the profit for the same amount of effort, remembering that currently characters can be invited into any tier and therefore the second character doesn't ever need money expended on them to get tier access.
Or abusing the glitch that allows players to repeat floors (which using a spare character is pretty much required for, though I won't explain how it works)
Those are the two that come to mind, although depending on how pvp matchmaking works it may also be possible to help yourself get free wins by multiboxing, which would also count as a bad thing.

I was not aware of the profit exploit there. Now I'm sure some newbies will attempt it after reading your post xD
u dont need 2 PC to play multiple accounts.
and even once a GM asked me to login and use both accounts on the same time, so I dont think it's illegal.
I've once tried getting two accounts before I realised that Mist Energy was tied to my PC unless I had spent money. Anyway I tied the site login soon after to my Steam account.
I also traded some items and money between accounts just for a test, on the same PC. I have a decently powerful PC.
There's no rule against multi-boxing. I've done it, but honestly there isn't much point. You get the same number of item drops no matter how many players there are; the only difference with multi-boxing is that you'll get double the crowns... for double the energy.
Multi-boxing is effective in other games because there is no cost associated with running a dungeon; in SK, when you run with two characters you're effectively paying twice as much.

The only reward I can see for multiboxing over just plain having "crafting alts" is to save time running the same dunegon at the same time. And dang if your computer can handle it since it's Java HAHA.
...If you go to the Sticky, the first thread in this and every other forum, you'll see a list of things you CAN and CANNOT do, both in the Forum AND in the Spiral Knights game.
Here's the Terms of Service that you agreed to as a quicklink if you have any further questions:
What do you mean by multiboxing? Its not like you can handle two games launched at once, if you want to use an alt for exploiting some stuff then i suggest you to not do so since there was no official statement on this and you might as well be banned for doing it. And generally the countermeasures from the game's mist energy system prevents you from abusing multiaccounting as long as you don't pay at least once.