Hey - Nice work! If I’m understanding your question, the closest I can think of in synfig is Linking a value. Like if you are fading one object by animating the value of “Amount” from 1.0 down to 0.0, you can then right-click amount, choose Export Value, name it “fade amount”. Then you can click on that shopping bag looking logo and choose “ValueBase Nodes”. Select the name of the value you made, such as “fade amount”. Then select the second object, right click Amount, and choose “Link,” and now that second object will do whatever amounts the first object is doing, even if you modify the first animation.
But I agree I don’t know a way to directly copy and paste as you describe.