Actually setting up a software environment that connects two participants in the same simulation, and have it run without any bugs is usually way beyond the capabilities of the psychology students running the experiment. Not to mention the bother of coordinating the different teamed up participants (“Did you click start? The other participant is waiting for you to click sta— oh no it timed out. Hang on I'll restart the pairing sequence…”).

If you participate in one of those studies on campus, you are facing a computer running a local bit of standalone software, or a webservice running a questionnaire.

Surely you could solve this problem quite simply by having both participants and the experimenter join a group chat using COTS software+accounts (Skype, Hangouts, Whatsapp...) pre-installed on the machines?

You would normally want to limit their input to a few form fields so the data can be analysed, rather than letting them have a conversation. Look at the ultimatum game for example.