Ornamental Mountain Ash

We have 2 of the above planted either side of a window about 4 feet from the house wall and about 8 feet apart. They are about 23 years old and 12feet tall. The branches currenly meet in the middle and they have gone above the single story roof line against which they are planted. Would it be in order at the end of the berry sesaon to give them an overall haircut by way of pruning about 3/4ft off all branches or would this have a detrimental affect on the plants life.