[TowerTalk] tower spacing

[w5jmw at towerfarm.net]

I have a hdx 555 and a ndx 455.I will be using one for hf and the other 
for vhf/uhf.My question is how close can they be to each other? I 
realize they should be as far apart as possible buton a lot and for the 
coax runs etc..what is a good distance? thanjks all w5jmw john