I guess you use the default mail base / file path? this will result in having two completely distinct downloads/mail bases (one for each account) and thus the behaviour

the main trick would be to use the same file base in both (all) accounts - the easiest way to do this would have been after installing TB when the inital dialog asks you for the mail base directory... as default TB uses "%APPDATA%\The Bat!\" as the mail folder - %APPDATA% being a XP variable which is user specific (I don't know the exact English path name... but it's a hidden folder in the standard documents and settings path)

however, you can do this later-on, too - as I'm not sure, how TB reacts on possible missing registry entries, I'd strongly advise to make a full backup before trying anything!

you can change the Mail Directory setting in
   Options/Preferences/System
  to point to an absolute path (e.g. C:\mail\)

  ATTENTION: This MOVES the mail directory structure
  to the given path! so is may well overwrite
  something there!

you can then use the same path in the second (Windows) account... BUT I guess it wouldn't be a good idea to also move the folders of that installation to the same place.... I'd try to delete the accounts (give a pseudo account a last) and THEN change the main mail folder - if you afterwards add a new account with the same name as the existing on (from the other XP account), TB will find and use the contents of those folders AFAIK... voila, you should be done...

THIS IS JUST THEORY for me now! as I DID NOT TRY THIS before in the given constellation, I again strongly advise to MAKE BACKUPs beforehand and ensure that no downloaded mail can get lost while trying (leave the copies on the server!)

hope this works?!

PS: assure the used folder is accessible (for reading and writing) for all relevant Windows accounts