Login to your main account. Go to your League of legends folder, locate League of legends and the folder Config. Inside this config-folder you find a file named PersistedSettings.json, copy this file to somewhere you always can find it. Make a new folder and paste your PersistedSettings.json there.Go out of all folders except the new one you made for your PersistedSettings.json. Login to your other account, go into your League of legends folder again, and once again locate the Config folder. These files are linked to accounts and not your computer. Take the file from your new folder, paste it into the Config folder and overwrite the current PersistedSettings.json.