Can a doctor write another script on the same prescription for medrol dose pack?

No it is against the law.

Most Drs dont write more than one prescription per document but I believe it is okay to put more than one Rx on a document as long as the drug is not a controlled substance. Controlled drugs must be written by themselves and have one prescription per document. Medrol is not a controlled substance. Most Docs only write one script per each document just as a "best practice" sort of thing and to avoid confusion and mistakes at the pharmacy. Regulations can be different from state to state as well.

If you mean add another item on the same prescription paper ... then yes, but it has to be the doctor who initially wrote the prescription.