qgis - Merge lines while adding attributes spatially

What I have:

A shapefile with multiple lines.

Example:

line 1: A -> B -> C with attributeX=5

line 2: A -> B -> D with attributeX=7

What I want

Lines between each nodes with added attributes.

Example:

A -> B with attributeX=12

B -> C with attributeX=5

B -> D with attributeX=7

(QGIS 2.0)