When to prune Amelanchier Lamarckii

Good evening,

I have got a couple of Amelanchier Lamarckii multi stem trees in my garden. They have been growing well for the past year and I'd like to shape them into a nice multistem shape with a lifted canopy at some point. I've included some pics of them.

When is the best time to do this? Should I wait till they a more mature?

Many thanks,
Ed