Usually. Depending on the position of the twins, the sonographer it is usually, but not always, possible to image both twins in detail and determine gender, whether they are monochorionic or dichorinonic, and to visualize their umbilical cords. However, this depends on their current gestational age. Optimal images may take more than one visit for an ultrasonogram.
Yes. Usually both twins can be examined in detail with ultrasound, but occasionally one is more difficult due to positioning. Rescanning a different day may reveal better details for the "hiding" twin.