There's a folder containing all of the UI settings for each of your characters in the My Documents\My Games folder. This includes UI positions, keybinds, macros, settings, basically any changes you've made to the default UI. Find the folder belonging to your main character (which you can do by checking the "Date Modified" property or looking for the log folder, which contains chat and battle logs), then copy its contents to the folder belonging to your alt (again, check the log folder and look for your alts names in the chat log).

Make sure to close the game while doing this.