FWIW Since this thread was linked to me!
I don’t believe it’s accurate to say that we don’t currently have a policy for overlapping installations (although probably it’s true that we don’t have an explicit policy written out). PEP3147 and PEP3149 were explicitly created to support overlapping installations and has been supported for 15 years now.
It would probably be good to actually write down what the actual promises we make for that (and if we want to abandon support for that and make no promises, that is also of course our prerogative as well 
).