Texture Mapping in Rhino

A curved surface can be mapped a gradation jpeg.

But a joined surface like a below picture can not be mapped the gradation jpeg.

Is it possible to map the jpeg texture on that joined surface?

Try with this example
Rhino 5 Mapping

You can use the curved surface to map onto the polysurface if you like.
Rhino supports custom mapping geometry.

you can… but it is not the most straight forward or user friendly procedure

you will need to swap the direction on one of the surfaces before you join

add a custom mesh to the objects, UNCHECK pack textures.

depending on the texture (gradient reflected or not) adjust your surface mapping so that U is only showing 0.5.