This doesn't mean separating them completely. Separate plants look like unnatural afterthoughts. A neighbor lined the front of his yard with a mound of topsoil and bark, and planted a heather every six feet. It looks just like the Civil War battlements I saw in Tennessee. Every time I look at his yard, I expect to see a sprawled trooper and a muzzle peeking over the mound or around one of the bushes.
Crowded shrubs look like a single mass.
Appropriately spaced shrubs will still merge but continue to display their individual shapes. Pruning back the shrubs is a temporary solution since the shrubs will only grow back to their usual size. A better solution would be to remove some of the shrubs, move them farther apart, combined with pruning to tidy and shape. Then add some smaller shrubs for contrast. Shrubs planted around trees for contrast may eventually grow into the trees, obliterating the line between middle and upper levels.