When you add a spline, a spline node tree is added for Front1 or Front2. The spline node tree includes both Spline and Axis nodes.
To add a spline node tree:
A spline node tree is added for either Front1 or Front2 in the schematic. It includes source and destination spline nodes, each parented to their own Axis node, which are in turn parented to a common axis.

(a) Spline node trees for one Front1 (Input1), and one Front2 (Input2) spline (b) Axis nodes for individual spline wireframes (c) Spline nodes for corresponding source and destination splines