It actually isn't possible, at least using the Views Forms functionality that we use to create the shopping cart form at /cart. The issue is that using a Views Forms field in a View turns the entire View into a form - as in, you get a new form element in the DOM that wraps around the entire View HTML. Because the cart contents checkout pane is already part of a form, you end up nesting a form within a form - which just doesn't work.
You have two basic options to work around:
- Alter your checkout form to embed custom quantity editing fields in that part of the checkout form. Difficult, not sure it's possible. (In the same vein, you could also figure out how to make Views Forms embeddable inside other forms - even more difficult, but probably more possible.)
- Use a custom block on /checkout to show the cart contents with a Views Form that saves updates asynchronously (not the default behavior). The idea there would be you'd have a form with quantity edit fields outside the checkout form that you'd be using to update the cart contents completely separate from whatever happens on the checkout form. I say you'll want to use AJAX to save the updates because you don't want to have someone submitting a cart update form after putting data into the checkout form elements below and losing data.
These two options work with the existing Views powered cart contents pane. If I had this scenario and owned the feature, I'd probably just create a custom checkout pane and eschew the Views powered form entirely.