Edit:
You DO NOT NEED STEAM IN ORDER TO MULTIBOX ROTMG!*
Realm of the Mad God page on our wiki
*
HOWTO: Multibox a flash game explains how to set up an ISBoxer compatible standalone exe for ROTMG!
Below this line is the original post about the Steam method:
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Okay. I have something available for someone to try right now, if they have multiple PCs and Steam accounts (Steam accounts are free and ROTMG is free, so this part shouldn't be the kicker). This works a bit differently because of Steam.
Inner Space build 5534 adds support for Steam games. Build 5534 is currently (as of this morning) the development build of Inner Space. To try it out, right click IS in the system notification area and select Patcher, check "Download development (test) patches", then close and restart Inner Space to let it patch up.
I VERY BRIEFLY tested the new support with a few games to make sure that the games launch and Inner Space seems to basically work. I am aware of some issues, like Terraria crashes if I try to play that, Portal 2 says "NO STEAM", etc. But, I didn't find any issues with Realm of the Mad God yet --
ROTMG recognizes my Steam profile and lets me play, so it'll be worth a try for anyone interested.
With all of that said... I hope to get this working on the same PC, either with the custom-built exe method or by somehow supporting multiple Steam logins.
So..
In order to try ROTMG with ISBoxer and Multiple PCs:
1. Launching the game: You're going to launch your ROTMG instance(s) manually, and here's how. You can add Steam.exe to Inner Space (via Add Game in the IS menu), call the "game" Steam, and launch Steam through Inner Space. Anything you launch with this Inner Space-launched Steam instance will be handled by Inner Space. So when you're going to launch your ISBoxer team, just follow this process and launch Steam through IS and ROTMG (or other game) through Steam. When you launch the game manually in this fashion, if it works correctly you should see FPS and Memory indicators in the upper left corner, and the title bar of the window should change to something like "is1 (ctrl+alt+1)"
2. ISBoxer Character Sets: Set your Game and Game Profile to Steam and the Steam Default Profile (created when you use Add Game). Put one Character in your Character Set, since you're only going to have one ROTMG instance per PC, tied to whatever Steam account you have active
3. Launching your Character Set: AFTER launching the game manually through Steam (which was launched through Inner Space), "Launch" your Character Set by the usual methods and ISBoxer will assimilate the window you have already launched.
Aside from these details, there is absolutely nothing different about this than any other game with multiple PCs, following the
Using multiple computers guide. (Note: Video FX views are not available from the separate PCs, only windows on the same PC)
That should be it, it should be 100% possible to multibox ROTMG with ISBoxer, right this very moment, with multiple PCs and the development build of Inner Space.