How can synchronization between the egg donor and the recipient be achieved during IVF with egg donation?

IVF with egg donation requires stimulating the donor's ovaries and retrieving her eggs, in synchronization with the uterine preparation of the egg recipient.

How is the coordination between the two achieved?

The donor's ovarian stimulation can be initiated on any day of her menstrual cycle and not necessarily on her second day of the cycle. While the endometrial preparation of the recipient should start on the second day of her menstrual cycle if she still has a period.
Therefore, the doctor starts stimulating the donor's ovaries on the day he starts preparing the endometrium of the recipient.