[TowerTalk] distance between monobanders

[Blair S Balden]

Hi all,
I have a 4-element 10 meter beam for my tower.  I also want to put up a 6-meter beam, and possibly one for 2 meters as well - all on the same mast.  I know there are a lot of experienced amateurs on this forum, so my question to you is: how much vertical spacing should I allow between the antennas?  Thanks in advance for any advice.
Blair, NP2F