The idea that Pinchas killed both Zimri and Cozbi at the same time with one spear is a highly miraculous feat. The Meam Loez actually lists twelve miracles that occurred at that moment when Pinchas successfully ended the plague.
He was able to kill them while they were still together. If they were separated, he would not have been allowed to kill them.
The second miracle was that neither Zimri or Cozbi protested when Pinchas came. Had they screamed, other Shimonites would have saved them.
Several of the miracles involved the spear itself. The spear was able to hold two people without breaking. It even lengthened on its own.
It was also miraculous that Pinchas exhibited such physical strength to do such an act.
Two final miracles were that they did not expire until they were taken out of the camp so that Pinchas, a Kohein was not defiled. And the last miracle was that their blood congealed and did not drip downwards.
Some say the מלאך המות came the men of Zimri's tribe that prevented them from hurting Pinchas.
We all know the story of Pinchas but maybe were not aware the extent of his heroic act and how he was helped from above. We must always be aware of Hashem's miracles and acknowledge them. Shabbat Shalom