Buxus can easily become dehydrated if not watered regularly (particularly during hot weather).
We recommend giving each buxus tree a bucket of water each everyday in hot weather. Many people believe that these plants do not need water if it rains, however, they often don't get much hydration if situated close to buildings and further the amount of rainfall is never enough, most rain water will run off the leaves and not in to the pot.
One Buxus is growing bigger than the other Buxus
A common issue is where one Buxus seems to spurt ahead in its growth compared to the other one. This can happen when positioned either side of a doorway.
Quite often the reason for this is the quota of sunlight and shade that the two plants receive during daylight hours. You'll find that one of them receives more shade than the other one.
To rectify this situation just alternate the two plants every few weeks. You'll then find that they will grow at the same speed.