Video Tutorial by Spline:
https://spline.design/tutorials/VSddFnDjPUA-how-to-design-and-use-components-in-3d-with-spline
https://www.youtube.com/watch?time_continue=40&v=VSddFnDjPUA&embeds_referring_euri=https://spline.design/&source_ve_path=Mjg2NjY&feature=emb_logo
Select Objects (in Outliner or 3D Viewport)
Right Click and Select Create Component (Command / CTRL + Shift + K)
Duplicate Component (Ctrl + D)
To Create 2nd Instance: Duplicate First Instance (Ctrl + D)