Sorry you're having trouble with the account login window, jeffjl. Before I give you a long list of things to try, a question: Does one (or both) of your accounts have a huge number of characters (including deleted characters) on it?
I have gotten "Account is in use" using both DarqUI and Default, so like you I don't attribute it to the custom UI. Whenever I get the error I have to give it a minute or so before I can log in again on any character on that account. It may be a simple matter of giving it a few extra seconds to release acct1 on the server, or whatever it's doing during that 60 seconds or so.
If that doesn't get acct1.char1 logged in, the next thing I would try is, before doing bullet point #4, click the acct2 tab then go back to the acct1 tab and choose char2. This will ensure the correct credentials get re-entered into the slots on the account login window (not the camp window). It's possible that the account/password slots on the account login window are getting fouled up when you get the error so clicking acct2, then acct1.char2, should refresh everything.
If it's still not recovering from the error message, I would try temporarily deleting eq2ui_loginscene.xml and eq2ui_mainhud_shutdown.xml from your custom UI folder. That will move the entire login procedure to Default. Use that for a few days and see what happens when you manually type creds into the window after the account-in-use error (Type /camp login or use the "Camp to Login" button since you won't have any of the custom stuff.) I get that error so rarely that it's hard to diagnose, and I'd be curious to know your results.