These will naturally grow into an upright oval to conical form. But it's not uncommon for more than one leader to develop. If you want, you can always clip back the secondary leaders to slow them down.

how about a pic ...

im concerned that trunks are underground .... they usually shouldnt be ... but it might be how they were propagated .....

when planting.. i would pull back the potting media.. and find out whats going on ...

i might even bare root them.. depending on what i find .... like severely root bound in the pot ...

big city location would help ... there are specific times to do this kind of root surgery ...

a primer on the process below ...

also ... if your GOAL is screening.. i dont understand why you want single trunk shrubs???? ... seems to me.. that would open the bottom up to eventually see thru it.. defeating your goal .... maybe a pic of the spot would also help ...

ken

https://sites.google.com/site/tnarboretum/Home/planting-a-tree-or-shrub