Let's start with the circular hole ( W_GEN_Beam Service Hole Circ.rfa ). When loaded this can be found under Generic Models.
Go to an elevation of the beam and drag and drop the circular hole family onto it. In this view the outline of the hole is shown in invisible lines, although you can still see and select them when you hover over them as is normal in Revit.Then using Opening By Face cut a hole directly on top of the invisible lines. Use the Pick Lines for the top half and lock the two together.Draw the bottom half using the Line Arc tools.Finish sketch. You now have your opening locked and aligned to the invisible lines in the family.
Thus ensuring the two are always in sync.