Want to run two accounts at once? The Core Fiesta launcher does it by itself - you do not need Sandboxie.

Steps

1. Open the launcher Settings and check that Multiple game clients is ticked (it is on by default).
2. Sign in with your first account and click Play.
3. Sign in with your second account in the same launcher and click Play again.

Both clients now run at the same time. The same account cannot play twice at once - the launcher tells you if you try.

Based on the original guide by HyperSpace on the Core Fiesta forums.