how do i prune my Japanese lilacs

I have 2 japaness lilacs that have never been prune. The form now is one of multiple trucnk small trees (20-25'). I thinkthey get just enough light to bloom. They make a great shield from the neighbors but I would likethem to be a better flower producer. Also can I prune them to tree form now that they are nolonger young?

 

Shading may be the problem, solution more light. Too big and far along to reduce to single trunks.