It will be mast mounted on a roof Tripod, with the thrust bearing between the Rotator and the first antenna, and guyed at four points on the thrust bearing. I know this is pushing the envelope, so maybe 3 feet separation would be less strain on the rotator.
How do you know that you're pushing the envelope with the proposed setup? I usually recommend that builders gain some understanding on the mechanics of their antenna/mast/wind load/mast material and the relative positions on the tower
of these parameters by using WD9P's instructional
mast calculator http://www.math.niu.edu/KARC/mast/
. You'll need the wind load specifications of your two antennas (from the manufacturer), the load ratings from your roof tower manufacturer and the type of mast material that you are using (aluminum or steel; wall thickness). I believe that you are in a 70mph region.
Using roof towers with multiple yagis can be risky. If you the least bit unsure of the safety factor then I would seek out a qualified professional engineer to run your numbers.
The wind forces on the bearings will be high. ... -Mike.
That's true Mike, but how 'high' is high? Two times, Three time as high? Without the above tool, it's just guesswork) - a more important engineering consideration may be how well the roof tower is affixed to the roof and how much lateral force is exerted at the tower feet - roof interface.
... This is the reason you see most spaced about 2-3 ft apart. ... -Mike.
I'm only familiar with the several dozen multiple HF
(I'm including 6M here) yagi configurations that I've personally seen, and probably another several dozen that I've seen pictures of (on QSL cards, etc.) and the vast majority of these had stacking distances between dissimilar yagis of between 5 and 10 feet. I've only see one or two situations that had three HF
antennas (example: M-square 20M4, 15M4, and 10M4) all on 6 feet of mast. So I guess it depends where you look.
GL, 73, Rich, K3VAT