This way is the long and laborious way of doing it. I use the same steam folder for all my idle accounts and have had 7 running from the same directory at once.
1: Download Sandboxie: www.Sandboxie.com
2: Open Sandboxie, click Sandbox->Create New Sandbox (name it SteamAlt or something)
3: Sandbox->SteamAlt->Sandbox Settings:
- Restrictions->Drop Rights->Untick 'Drop rights from Administrators and Power Users groups'
- Resource Access->File Access->Direct Access:
-> Add Program (Steam.exe)
-> Add directory for Steam.exe
4: Sandbox->SteamAlt->Run Sandboxed->Run Windows Explorer
5: In Sandboxed explorer, navigate to Steam directory and run Steam.exe
6: You can now sign in to whatever account you like, even if Steam is already open, and can load TF2.