Smoothing intersected bezier curves

I have found some blender 2.4 tutorial about making trees. The problem is how to make that nice fill between duplicated curves, just like on the picture: I’ve been trying to configure somehow a bezier curve settings in properties panel and results are still like on left picture. Does anyone know what should I do?

Why do you think that is done with Bezier curves? No T-junctions are allowed in Beziers - anywhere AFAIK - and Blender’s NURBS implementation does not allow them yet (fingers X’d). Most likely done with meshes converted from curve objects before joining.

This effect is part of this tutorial: It looks like it’s made automatically using early version of sapling plugin.