Could i ride rollercoasters 6 weeks after acl and meniscus surgery?

Without accident, impact to the knees is negligible with the ride so I do not think it is contraindicated.

Discuss with your surgeon. Depends on the surgery performed, and how long they want to protect the healing. Depending on the force of the rollercoasters, and the size of the seats, and the walking involved, may cause more pain and swelling of the knee and slow your recovery.