If you install a debug built APK on an Android device, only one can be installed.

Only one version of the same application can exist.
The word "identical" here means that the application package names are the same.

On the other hand, if you change the name of the application package, it can exist together (although to be exact, it makes you recognize it as a different application).
This is the <manifest> element package attribute

<manifest package="com.example.project".>

by changing the .

Note: http://developer.android.com/intl/ja/guide/topics/manifest/manifest-element.html#package

The package name definitions your application's identity, so if you change it, then it is considered to be a different application

Note that if the name of the activity or class of service is written in simplify, additional modifications to the FQDN notation may occur.

The general explanation for Android development is as above, but for Monaca, it would be better to set it up on the settings page where you can choose from the menu.