A stage in the Wales Rally GB was stopped after a 14-year-old boy and a 39-year-old man were injured.
Police officers are working on the theory that the pair, believed to be nephew and uncle, were struck by flying debris from one of the cars.
They were taken to Princess of Wales Hospital, Bridgend, for treatment after the incident near Port Talbot during stage two of the rally at 1130 GMT.
Competitors were directed to the next stage while an inquiry took place.
The spectators were said to have suffered minor injuries.
A spokeswoman for South Wales Police said: "Police are at the scene and are working with Wales Rally GB personnel in a joint investigation into the circumstances of the incident."
Clerk of the course Fred Gallagher told BBC Radio Wales: "With lots of spectators and our own marshals, there are lots of reports to get.
"We're trying to assimilate all those reports at the moment and get a definitive idea of what happened."
He added that the spectator area at the point where the spectators were injured was on top of a high bank and there were no barriers.
"Virtually nowhere in rallying do you have physical barriers," said Mr Gallagher.
"There is a spectator forbidden zone between the edge of the roadway and the banking.
"As you can imagine we're just trying to find out exactly where the people involved were."
In 2005, co-driver Michael Park died in a high speed crash in Bryn Forest near Port Talbot.
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