CRA responded
The CRA confirms that it will accept a single Form GST159 to be filed for multiple assessments under objection. To enable us to determine the validity of the objection, it is important that the periods covered and the information on the assessments (notice number and date) be properly identified on the Notice of Objection. If there is insufficient space on the form, a separate sheet can be attached. … Whenever there is insufficient space on the Notice of Objection form to provide facts and reasons for the objection or the assessment information, you can always attach a separate sheet(s). …
Where CRA issues multiple GST/HST assessments they can be objected to on a single Notice of Objection. The required particulars can be provided in attachments thereto.