bundler env problem

A_Gemfile

# A/Gemfile

gem 'thor'

A_test.rb

# A/test.rb

require 'rubygems'
require 'bundler/setup'

system("cd ../B && bundle show")

# Now this system command shows:
#
#   Gems included by the bundle:
#    * bundler (1.0.10)
#    * thor (0.14.6)
#
# which are gems for directory A, not B
#
# If you however comment out "require 'bundler/setup'", it will show proper output, for directory B that is:
#
#   Gems included by the bundle:
#     * bundler (1.0.10)
#     * git (1.2.5)
#

 
# UPDATE: Passing BUNDLE_GEMFILE="Gemfile" seems to help for showing them, 
# it still won't install them properly however (tried BUNDLE BUNDLE_APP_CONFIG too).
#
# How does that bundler even know that it is run from within another application using bundler? How?

B_Gemfile

# B/Gemfile

gem 'git'