I wanted to split up the config file for my middleman site. They dont make it easy for you because they use instance_eval to read the file.

The following technique will allow you to load additional config files correctly.

# config.rb

eval File.read('config/blog.rb')
eval File.read('config/search.rb')
eval File.read('config/proxy_pages.rb')
eval File.read('config/deploy.rb')