It happened on the northbound carriageway between junction 45 (Whitkirk) and junction 46 (Colton) at around 1.25pm on Saturday.
Police today said that a Vauxhall Vivaro had collided with a Vauxhall Zafira, which was travelling northbound in lane one.
A spokesman said: "The driver and front seat passenger of the Zafira suffered serious injuries and were taken to Leeds General Infirmary for urgent medical treatment.
"They both remain in hospital."
The driver of the Vivaro stopped at the scene but police are still seeking witnesses to the collision.
Anyone with any information is askd to contact the police via 101, quoting log number 0981 of 3/6.