The new style helmets that we fly with don't have dual visor capability unfortunately. Tree top is right, it normally is not an issue, but on a couple occasions it has been for me.
MDC = Mild Detonating Cord. It is the squiggly looking grey wire looking stuff you can see on the top of the canopy of T-45's, Hawks, and Harriers. I'm not an expert on it, but it basically destroys the canopy prior to the seats going up through during ejection. The outside of it has a coating of lead which melts the canopy around the cable, and in past ejections, it has injured folks who haven't been properly covered (arms and face) as the stuff comes raining down on them in their seat. I have heard that it was properly designed for the Harrier, and then retrofitted into the Hawk and T-45, where it is essentially way overpowered which causes this problem. I won't get into details of past mishaps, but it has caused serious injuries to several folks who have had to punch out. Good reason to wear your O2 mask with the visor down whenever the seat is armed, and to keep your sleeves rolled down and gloves on.
