I don't see why you can't add more than one activity to the same application.
See for instance the Devoxx application. The Android manifest contains several activities:
<application android:label="Devoxx" android:name="android.support.multidex.MultiDexApplication" android:icon="@mipmap/ic_launcher">
...
<activity android:name="javafxports.android.FXActivity"
android:label="Devoxx"
android:launchMode="singleTop"
android:screenOrientation="portrait"
android:configChanges="screenSize">
...
<intent-filter>
<action android:name="android.intent.action.MAIN"/>
<category android:name="android.intent.category.LAUNCHER"/>
</intent-filter>
</activity>
<activity android:name="com.gluonhq.impl.charm.down.plugins.android.NotificationActivity"
android:parentActivityName="javafxports.android.FXActivity">
<meta-data android:name="android.support.PARENT_ACTIVITY"
android:value="javafxports.android.FXActivity"/>
</activity>
...
</application>
These are intended to work with the Charm Down plugins (Permissions, PushNotifications, LocalNotifications, Barcode Scanner), that have defined several specific activities.