You say young trees can not Pollard. How old is young I have 2 silver birches planted 2013 too tall now Can they be topped
You can pollard an older tree, but it will be slow to regrow and more susceptible to disease. Also, once you start, you have to keep it up. For true pollarding, you wouldn't prune a branch older than two or three years. Pruning older limbs can be damaging and cause decay.
Armstrong nursery says that I can remove one of the trunks of the tree without damaging the tree. I just want to confirm what your thoughts are. This tree is in Temecula Ca.
If the trees are young and healthy it should be OK to separate them. If the tree is mature it may not survive the separation. Do it when the tree is dormant to help prevent disease and insect injury.
I planted 3 golden treasure dwarf birch shrubs 5/2021. They get about 4 hrs morning sun. All look totally woody with only a few leaves on top.
The leaves look spotty, like they have a fungal infection. The best thing you can do is prevent it next year. Clean up all the fallen leaves and dispose of them in the trash. Water from below; avoid wetting the leaves. The soil looks pretty dry, so you may want to increase the water.
The shrub prefers part shade. So if you have it in full shade you may want to move it.