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.