The only ways to share addon settings/data between characters are:
(1) Use a symlink/junction to make both of your accounts' folders point to the same real folder:
Code:
cd "C:\Games\World of Warcraft\WTF"
mklink /j "654321#1" "123456#1"
... where 123456#1 is the name of your main account, and 654321#1 is the name of your secondary account. Exit WoW and delete the real folder for the secondary account before running this command. With this method, after it's set up, all settings for the game and all addons will be shared by both accounts, and you never have to do anything else. I'd suggest using this method, and used it myself when I had two active accounts.
The above commands are for Windows Vista or higher. If you're on Windows XP, you'll need to download the Junction utility from Microsoft (google "site:microsoft.com windows xp junction" to find it probably). For all versions of Windows, you can get to the command line console by pressing Win+R (or going to Start Menu > Run), typing "cmd", and pressing Enter. If you're on Mac or Linux, the procedure will obviously be different.
(2) Manually copy/paste the addon's saved variables file between accounts every time you switch accounts. With this method, you'll lose data if you ever log onto both accounts at the same time, and you'll lose data if you ever forget to copy the file before switching.
(3) Find/write an addon that syncs the data in-game over the addon communication channel. With this method, you have to tell the addon the names of all your alts on both accounts, and it only works if you always log into both accounts at the same time (otherwise the data for one account will be out of date when viewed from the other). Also, it requires a decent amount of code in the addon, and only works for that addon.