Do I have to build two sis files for Anna and Belle or one is enough?

My app only uses qt qml javascript and qt quick symbian components.

Do I have to build it twice, one for Anna and one for Belle, or I can build only once for them both?

Thank you.

Assuming that with Belle Nokia has retained backwards compatibility, then make an Anna build and it should also work on Belle. Test it yourself, of course, before releasing your app.