2 seperate installation 1 theme same server

i got 2 separate installation on the same folder
each in their own sub folder – they use the same theme
which i copied twice.

Every time i edit a file i need to do it twice and its
a real hassle… How can i define so that they both
use the same theme – a shared theme but a different

I have no “initial code” to share since i don’t know
where to begin so an example code would be much appreciated

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