You can align to a specific dimension (horizontal or vertical) using the "Distribute Spacing" selection in the "Align" tab.

![enter image description here][1]

*Note: If the text box does not display, in the upper right of the panel is a dropbox... click it and choose "Show Options".*

 1. Select the objects you want to distribute.
 2. Single-click on one object to set it as the anchor (aka key) object.
 3. Enter the amount you want them distributed. 
 4. Click on either the Distribute Vertically button or the Distribute Horizontally button.

You can also use this to place two objects exactly next to each other; just set your spacing to "0".


  [1]: https://i.sstatic.net/cY4Ta.png