How to add multiple widgets in the same app?

You need a receiver definition for each type in your manifest file like:

    <receiver android:name=".MyWidget" android:label="@string/medium_widget_name">
        <intent-filter>
            <action android:name="android.appwidget.action.APPWIDGET_UPDATE" />
        </intent-filter>
        <meta-data android:name="android.appwidget.provider"
            android:resource="@xml/medium_widget_provider" />
    </receiver>

    <receiver android:name=".MyWidget" android:label="@string/large_widget_name">
        <intent-filter>
            <action android:name="android.appwidget.action.APPWIDGET_UPDATE" />
        </intent-filter>
        <meta-data android:name="android.appwidget.provider"
            android:resource="@xml/large_widget_provider" />
    </receiver>

This would allow you to have the same AppWidgetProvider class be used for multiple widgets, with different widget names and different sizes defined in the <appwidget-provider> XML.

Now if you need more differences in your widgets than what is in the <appwidget-provider> XML I would create a base widget class that implements all the common behavoir between the different types:

public abstract class MyBaseWidget extends AppWidgetProvider

And then each of your concrete implementations could extend MyBaseWidget. Then in your manifest file you would have a receiver definition for each of your concrete implementations like:

    <receiver android:name=".MyMediumWidget" android:label="@string/medium_widget_name">
        <intent-filter>
            <action android:name="android.appwidget.action.APPWIDGET_UPDATE" />
        </intent-filter>
        <meta-data android:name="android.appwidget.provider"
            android:resource="@xml/medium_widget_provider" />
    </receiver>

    <receiver android:name=".MyLargeWidget" android:label="@string/large_widget_name">
        <intent-filter>
            <action android:name="android.appwidget.action.APPWIDGET_UPDATE" />
        </intent-filter>
        <meta-data android:name="android.appwidget.provider"
            android:resource="@xml/large_widget_provider" />
    </receiver>