Multi-Boxing

Multi-Boxing is when a single person runs multiple clients on one or more computers in order to control multiple characters in a game.

An excerpt from the Community Guidelines about this:

3rd party Software, Multi-boxing, and Unfair play

The OPP server has strict guidelines against the use of 3rd party software to gain unfair advantages in-game.

Any automation or multi-client control software is forbidden in all cases.
The simultaneous operation of more than 5 accounts is prohibited.
Registering accounts with falsified, invalid, or unreachable email addresses, or those that are not owned by the player, are prohibited.

For any of these offenses, the OP Team reserves the right, in any case, to use discretion in all punitive actions.

Violations of any of these offenses can result in an immediate and permanent ban of the affected accounts.

Windows Setup

Video guide courtesy of JimbozGrapes:

Steps:

Get Perpetuum Steam Client (https://store.steampowered.com/app/223410/Perpetuum/)
Download and install (default steam directory)
Ensure the presence of a "steam_appid.txt" file in game folder (should be "Perpetuum" folder after using "Open Local Folder option) with content: 223410
Make a copy of the folder Perpetuum somewhere else
Login to Steam (you must now be logged in to use the client)
Run the Perpetuum.exe in each of the folders (make shortcuts, etc as you like); you can also add the shortcuts to Steam as "Non-Steam" games, like in Linux Setup below (step 7).

Disk Space Optimization

To save space you can use a symbolic link for the Layers.gbf and Perpetuum.gbf files. This way you only need to store one copy of the file no matter how many clients you are running.

On Windows, this can be done with the mklink command, or by using the "Create Shortcut" option in the right click menu.