Create a project with two web modules, keep all settings default.
In Run configurations dialog add two local tomcat configurations: "weba" configuration to deploy only "weba" module, "webb" configuration to deploy only "webb" module.
Edit field "Application context" of deployed module in each configuration. The value of this field is typically appended to "Startup page" field of the configuration. But in this case this appending happens only for one configuration, and does not happen for another.