Dehydration
Buxus can easily become dehydrated if not watered regularly (particularly during hot weather).
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.