One can use foreman to start multiple processes. Add foreman to your Gemfile, and then create two files:
Procfile:
worker: bundle exec foreman start -f Procfile.workers
Procfile.workers:
worker_1: QUEUE=* bundle exec rake resque:work
worker_2: QUEUE=* bundle exec rake resque:work
The same technique can be used to run a web server alongside some workers.
NOTE: while many state success using this approach, I would not suggest to use it outside of some experiments, mostly because of the risk to run into RAM limitations on small heroku instances; and once you pay for the heroku service it is probably easier to just spin up a dedicated worker machine anyways.