If you want to have two addresses at the top of the letter, with the left hand one *left aligned* and the right hand one *right aligned*, you'd find it easier to create a table with two columns and as many rows as you need (Table/Insert/Table). Highlight one of the columns by hovering your cursor above the column until you see a thick black arrow, click once and that column should turn black, then click on Format/Borders & Shading/Borders, then click on none. This will hide the border around the cells of the table. Then click on the *align left* button (or do it through Format/Paragraph/Indents & Spacing/left alignment). Repeat for the other column but make this right aligned this time.
When you type in the addresses you will find that the left hand side one is left aligned and the right hand one is right aligned.
To move between the cells of the table, just press the tab key (it's the one with dual horizontal arrows, to the left of the letter Q). If you want to increase the table by one or more rows, just click in the last cell and then press the tab key... this will add another row below.
After you have inserted your addresses, just click below the table and you should find that you are back in left align mode.