Do both Front Axle half shafts have to be replaced when only 1 CV boot leaks?

Are you talking about the two half shafts ( Inner ) and ( outer ) on the same side ? If so they are usually done together because of cost of new complete shaft is less then repairing one part. However you don't have to replace the drivers side just because the passenger side is bad. They also have repair boots, new grease can be added and repair boot installed on just the part that is bad if the cv joint it self is good. Hope this helps you.