advice two almost identical controllers

you could try some routing. add to routes.rb before the last two
automatically generated routes

map.invoices_in 'in/invoices/:action/:id',
  :controller => 'invoices', :type => 'in'
map.invoices_out 'out/invoices/:action/:id',
  :controller => 'invoices', :type => 'out'

so now if you do

http://localhost:3000/in/invoices/show/10

in your invoices controller:

def show
  if params[:type]=='in'
     # handle incoming invoices
  else
     # handle outgoing invoices
  end
end

in your view, you can use the named route

<%= link_to 'incoming invoice #1',
invoices_in_path( :action=>'show',:id => 1) %>

and it will generate

<a href="in/invoices/show/1">incoming invoice #1</a>

invoices_in_path and invoices_in_url methods are automatically created
for you because of his

map.invoices_in 'in/invoices/:action/:id',....