“ Moreover, theresearch found that when vertical wind turbines...

“ Moreover, theresearch found that when vertical wind turbines are set in pairs,they can increase each other’s performance by up to 15%. “To gethe details onhow and whent hat does or does not happen you had to go here as well

https://www.sciencedirect.com/science/article/pii/S096014812100344X

In there we findthat the boost happens under these condtions
“For theconfigurations analysed, pairs of VAWTs exhibited a 15% increase inpower output compared to operating in isolation, when the secondrotor was spaced three turbine diameters downstream and at an angleof 60° to the wind direction. “
Which pretty stonglysuggest to me that in some other wind orientation there is strong windshadow.

As the boost seems like it must must have gotten the extra effective wind strength from somewhere.