Oo. Could very much be it. Reinstalled AC/Decal, verified, reinstalled ACDC. Tried, no dice. Then:
1. Launched ACDC
2. Disabled Auto-Port, manually set to 9002
3. Launched WhereAC - no path in use, Current Port: 9002
4. Clicked "Find Path"
5. Login 1, started clicking Get Path -- nothing until it got past launcher. Upon "Connected/Patching Done!" splash screen, WhereAC showed C:\Games\Asheron's Call
Groovy.
6. ACDC, switched to 2nd account - WhereAC shows Current Port: 9008, and C:\Games\Asheron's Call as in use.
7. Clicked "Find Path"
8. Login 2, started clicking Get Path -- goes to Asheron's Call Launcher, starts updating launcher. Update completes, it tries to log into server, and poof - "There is already a client running..." error message.
When I reinstall ACDC and I launch my 2nd account for the first time (after copying files within ACDC setup), "Asheron's Call Launcher" splash screen does do an update. So, the launcher's at least being run from the 2nd directory (I assume) - It creates ACDCClient.exe in the new directory once I try to login from the second account, but perhaps it's still trying to run acclient from the main directory.
It's some sort of progress, I'll take it

tyty!
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The Fury
Rubiss/Ten-Toes/Three-Toes/Dash Cunning