[TowerTalk] 40 meter and HF beam on same tower

[K9OQ]

I have a HDX 555 with a TH11DX on it now.  I am thinking of putting a 3
element Hy Gain 40 meter antenna above it.  how much spacing do I need
between the two to avoid interaction.  The TH 11 DX works great right now
and I don't want to detune it.  Also the 40 meter if it were 15 feet above
would be about 70 feet off the ground which I'm thinking should be good.
Comments please.