For many journals the referee is asked to tick a box when they submit their report to indicate whether or not they (1) allow the report to be used for another journal and (2) whether their identity may be disclosed to the editors of that other journal.

Given this practice, the answer to question 1 would be a "yes". Whether or not the report is then actually transferred from journal X to journal Y (anonymously or with the identity of the referee disclosed) would be something journal X decides based on whether or not the referee authorized them to do so.

Concerning question 2, I don't think you need permission beforehand from journal X when you inform journal Y of the existence of a detailed report.

This complication will hopefully become a thing of the past, when more and more journals migrate to a practice where referee reports are public documents, even if anonymous.