You could use a “drill” to cut the holes in both groups using the Solid Tools. You could also model the hole in one part and use that as a reference for modeling the hole in the other part.
Edit one group, select the hole geometry and move it. Then open the other group and repeat the process.
To help achieve your goals, some time spent at the SketchUp Campus and at the SketchUp - YouTube channel will be very worthwhile. Both sites are from the SketchUp team. On the YouTube channel, pay attention to the Square One Series. It covers the basics for each tool.
Thanks RLGL - I have been picking through various videos but probably not going about it in a systematic way. Your guidance to the Sketchup Camus and Square One series I think will prove to be the more solid foundation I need.