2 yagis, 2 towers, one question

[VK3TJK]

Hi Guys - I am planning a 2 tower project - too many large yagis for one tower! - The two towers are freestanding and will be 80 feet each - Im wondering is there a distance spacing between such towers that is recommended ? Obviously as far away as possible!!?
There will be essentially very large HF monoband yagis at the same height facing one another, at certain times - I was thinking 100 feet spacing - Does anyone have any thoughts or advice for me?

Thanks
Tim

[PD2R]

At the PI4DX contest station we have two free standing rotatable towers some 130 feet appart. In one tower we have a 6 el for 15 meter and in the other there is a 4 el for 40 meters. Sometimes, especially when the antenna's are facing eachother, there is some interference. The use of good high and low power bandpass filters reduces interference significantly and is considered a must have when you want to use both towers/antennas/bands at the same time.