Thank you for using Bitnami. That's because both stacks were configured to use the same ports. Our installers test the default ports before installing the application to know if they are free or not. If they aren't, it will ask you to set a different port. For example, if your MAMP installation is using the 8080 port in Apache, MediaWiki will ask you to use the 8081.
If you didn't make any change in the MAMP installation, I suggest you uninstall it and then install it again while the MediaWiki installation is running. That will configure the second installation properly and you won't need to change anything in the configuration files.
Happy to help!
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