Ah, I've had a 2nd steam account for awhile, I plan on passing it off to my nephew whenever he gets internet on his computer. So I randomly purchase games for that account too.
Been playing TF2 a lot so I figured I could milk the drop system by swapping accounts when I play.
Anyway, getting items from one account to the other has required me to trade through a friend, swap accounts, trade again...
a slight pain for me but I feel bad getting someone else involved...
So I found some info at SPUF. An old thread that had 20 some posts. Posted an issue I had and an admin must've deleted it because it's not there anymore and the link I saved takes me to a 'you don't have permission page'. Which is funny because they never deleted it before.
Luckily they didn't delete the link that had the most info. So I'm posting it here for anyone interested.
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this will let you run a steam account with TF2 in the background, while running your regular steam account and playing TF2 or other games.
So no leaving the comp on all night to idle, just do it at the same time.
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DL Sandboxie.
http://www.sandboxie.com/
1. Copy your steam entire steam folder to desktop (takes a while, try to leave out unnecessary files, just stick to tf2 core files).
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the best way to do this is make a new Steam Folder somewhere apart from regular steam. Select all contents EXCEPT SteamApps folder from your regular steam folder. Copy them into the new Steam folder.
Open SteamApps and copy the user folder for your new account (if created already) and put that into your new Steam/SteamApps folder.
(I had to re-dl TF2 as copying the files and trying to start up told me to DL again) (My Orange Box code had already been used so I couldn't instal from there

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2. Create new sandbox.
3. Edit sandbox settings: unclick drop rights; add direct file access to steam.exe and steam folder (both from the one you copied to desktop)
4. Run sandbox in new windows explorer.
5. Open steam.exe from desktop.
6. Enjoy.
I believe that's about all there is to it. Works for me.
EDIT:
I can't find the thread that helped me figure this out, but as luck would have it I happened to copy everything that the OP said and still have it! So word for word:
1: Download Sandboxie: http://www.Sandboxie.com
2: Open Sandboxie, click Sandbox->Create New Sandbox (name it SteamAlt or something)
3: Copy your entire Steam directory (selectively copy only TF2 related files from SteamApps to save space), and place it in a new folder somewhere
4: 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, pointing to your copy of the Steam directory (best not to use the original Steam install, in case it conflicts with other running copies)
5: Sandbox->SteamAlt->Run Sandboxed->Run Windows Explorer
6: In Sandboxed explorer, navigate to copied Steam directory and run Steam.exe
7: You can now sign in to whatever account you like, even if Steam is already open, and can load TF2.
CREDIT: Anonymous original OP
This worked beautifully for me.
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NOTES:
I had an issue with 'mouse emulation' , there's a check box in sandboxie somewhere, the error will lead you to their web page. Must be updated because the button isn't quite where there site says, but it's there if you look hard enough.
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I also found it's best to have at least one of the TF2's in windowed mode. When I traded and minimized TF2 the other TF2's GUI was screwy and I couldn't click the Trade buttons. Windowed mode cured that issue.
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Unfortunately there was some info about running the second version of TF2 in txt mode only, no sound etc... to cut down on performance issues. But this was all in the post that has been deleted.
Ah, here's a blog with that info
http://keyfox-blog.net/?p=1463
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I saw a slight hit in my ping while having an idle server minimized and playing on my other account, but it was nothing major.
I didn't see a performance impact and have 'sound off when minimized' checked in TF2 options.
I got 2 drops on each account last night, so it works. Also had it running while playing L4D2.