conanfile.txt

[requires]
middle/test
middle2/test

libjpeg-proxy.py

from conans import ConanFile

class ProxyLibjpeg(ConanFile):
    name = "libjpeg-proxy"
    version = "any"
    
    options = {'libjpeg': ['libjpeg', 'libjpeg-turbo']}
    default_options = {'libjpeg': 'libjpeg'}
    
    def requirements(self):
      if self.options.libjpeg == 'libjpeg':
         self.requires('libjpeg/9d')
      elif self.options.libjpeg == 'libjpeg-turbo':
         self.requires('libjpeg-turbo/2.0.5')

libmiddle.py

from conans import ConanFile

class LibMiddle(ConanFile):
    name = 'middle'
    
    def requirements(self):
       self.requires('libjpeg-proxy/any')
       self.requires('libjpeg/9c', override=True) # In case 'libjpeg' is choosen
       self.requires('libjpeg-turbo/2.0.4', override=True) # In case 'libjpeg-turbo' is choosen

libmiddle2.py

from conans import ConanFile

class LibMiddle(ConanFile):
    name = 'middle2'
    
    def requirements(self):
       self.requires('libjpeg-proxy/any')

libmiddle3.py

from conans import ConanFile

class LibMiddle(ConanFile):
    name = 'middle3'
    
    def requirements(self):
       self.requires('libjpeg/9c') # Only libjpeg supported for whatever reason

run.sh

conan create libjpeg-proxy.py libjpeg-proxy/any@
conan create libmiddle.py middle/test@ --build=missing
conan create libmiddle2.py middle2/test@ --build=missing
conan create libmiddle3.py middle3/test@ --build=missing

conan install conanfile.txt --build=missing
conan install conanfile.txt -o libjpeg-proxy:libjpeg=libjpeg-turbo  --build=missing