Gary,
The 'best' separation distance is as much as you can get. After that,
it's more a practical thing, use what you got, do what you can. Will
that result in the 'best'/'perfect' situation? Of course not, but it
will work in the majority of cases.
A very rough ROT (Rule Of Thumb) is that UHF antennas require less
separation than VHF antennas, which require less separation than HF
antennas.
- 'Doc

Absolute best distance between antennas is at least 1 mile... in all
directions. I'm still working on mine.