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I have two bayed and used 75cm spacing, it is not that critical because i tried various different spacings 50-100cm when i first got them with only a small change in main lobe performance, mainly differing levels of side-lobes.
The larger the spacing the more the gain but at the cost of stronger side lobes...
Related article for VHF yagi stacking etc maybe interesting read: http://www.grantronics.com.au/docs/StkYagis.pdf
Slightly negative comment from me!
The maximum additional gain you will get from stacking two antennas is +3dB
This assumes no loss in the two way splitter, antennas with an identical impedance, identical feed cables etc.
All this is very unlikely so the best improvement you will get is say +2dB
It's really not worth all the effort. It would probably be worth putting up a stack or box of four but not two.