PBEM on same PC
From Solium Infernum Wiki
You can setup a PBEM game where two people are on the same PC, but they will each need a unique serial number (so you'll have to buy two copies of the game).
All game data is saved in the SoliumInfernumGame directory in the "My Documents" folder, so to switch players, you will need to switch that folder. One way is to just have two different users logins setup, that way each person has their own "My Documents" folder, and therefore their own SoliumInfernumGame folder.
The other way is to manually rename the folder. For example, John installs the game, enters his registration number, and then exits. John can now rename the SoliumInfernumGame to SoliumInfernumGameJOHN. Jane then starts up the game (no need to reinstall) and since the SoliumInfernumGame folder no longer exists, is asked for her registration number. She enters it and then exits the game. Jane renames the new SoliumInfernumGame folder to SoliumInfernumGameJANE. From that point on, when they want to play, they rename their respective folder to SoliumInfernumGame, take their turns, then rename the folder back to their personalized folder name. If you happen to know how to do batch files, all this switching can be somewhat automated so it is reduced to clicking on an icon.