You link a spline source node from Input1 to a spline source node from Input2 to create interpolation between them for a morph. Destination splines are no longer used and cleared from the schematic. The source of the Input2 spline node tree becomes the destination of the Input1 spline node tree.
You can create as many relationships between Input1 and Input2 spline nodes by linking them in the schematic, as there are features to correlate between the clips.
The splines are linked to each other, as indicated by the blue dotted connector lines. The destination splines, and their respective Axis nodes, are removed from the two spline node trees. Once Spline nodes are linked, you can set an interpolation value to determine the percentage that the feature defined by the Input1 spline mixes with the feature defined by the Input2 spline. See Interpolating Features.
The spline nodes are unlinked, and the destination splines and their axes reappear for each of the affected spline node trees.