by looking at the source (I have not tested if it works) looks like there is a method called
Foundation/Application.php
protected function normalizeCachePath($key, $default)
{
if (is_null($env = Env::get($key))) {
return $this->bootstrapPath($default);
}
return Str::startsWith($env, $this->absoluteCachePathPrefixes)
? $env
: $this->basePath($env);
}
which is called when requesting the config.php and other cached files, so you can set environment variables on the server, for each instance run
APP_CONFIG_CACHE=app/caches/site1/config.php APP_CONFIG_CACHE=app/caches/site2/config.php
this is of course if each site runs on its own environment.
Env class === Support/Env.php