If there is a quality shoulder then the law tells them to stay on the shoulder.
This is completely separate from single file vs. side by side. Just stay on the shoulder.
Even single file in the lane would break the rule.
If the shoulder is nearly the width of a full lane, then it would fit for them to be side by side entirely inside the shoulder, and that would be allowed too.
Sometimes, as a secondary effect, this rule blocks being side by side. But only when the shoulder is very specific widths. It's not the point of the rule.
This is completely separate from single file vs. side by side. Just stay on the shoulder.
Even single file in the lane would break the rule.
If the shoulder is nearly the width of a full lane, then it would fit for them to be side by side entirely inside the shoulder, and that would be allowed too.
Sometimes, as a secondary effect, this rule blocks being side by side. But only when the shoulder is very specific widths. It's not the point of the rule.