gistfile1.rb

namespace :test do
  task :double do
    Rake::Task["rake:test:units"].invoke
    Rake::Task["rake:test:units"].invoke
  end
  # etc
end

gistfile2.txt

[ joshuaclayton ~/Sites/jarob parallel_test ] $ RAILS_ENV=test rake test:double
(in /Users/joshuaclayton/Sites/jarob)
/usr/local/bin/ruby -I"lib:test" "/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake/rake_test_loader.rb" "test/unit/clearance_mailer_test.rb" "test/unit/customer_department_test.rb" "test/unit/customer_test.rb" "test/unit/list_item_test.rb" "test/unit/observers/product_template_observer_test.rb" "test/unit/presenters/customer_presenter_test.rb" "test/unit/presenters/customers_presenter_test.rb" "test/unit/presenters/list_item_presenter_test.rb" "test/unit/presenters/list_items_presenter_test.rb" "test/unit/presenters/product_list_presenter_test.rb" "test/unit/presenters/product_lists_presenter_test.rb" "test/unit/presenters/product_template_presenter_test.rb" "test/unit/presenters/product_templates_presenter_test.rb" "test/unit/presenters/product_variant_presenter_test.rb" "test/unit/presenters/user_presenter_test.rb" "test/unit/presenters/users_presenter_test.rb" "test/unit/product_list_test.rb" "test/unit/product_mounting_method_test.rb" "test/unit/product_template_department_test.rb" "test/unit/product_template_test.rb" "test/unit/product_tier_test.rb" "test/unit/product_variant_test.rb" "test/unit/sentinels/customer_sentinel_test.rb" "test/unit/sentinels/product_mounting_method_sentinel_test.rb" "test/unit/sentinels/user_sentinel_test.rb" "test/unit/tiered_price_test.rb" "test/unit/user_test.rb" 
  * DEFERRED: A customer instance should validate logo. 
  * DEFERRED: A product variant instance should validate installation guide. 
Loaded suite /usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake/rake_test_loader
Started
..............................................................................................................................................................................................................................................................................................................................................................................
Finished in 56.403977 seconds.

366 tests, 488 assertions, 0 failures, 0 errors
Loaded suite /usr/local/bin/rake
Started

Finished in 0.000686 seconds.

0 tests, 0 assertions, 0 failures, 0 errors