I didn't take the PMI-ACP exam, but — based on what I read on the PearsonVUE website — it looks like this is not allowed. In fact, here is what I found in one of the FAQs:
For PMI-ACP and CAPM exams, breaks are not allowed during the exam for any reason and leaving your desk will invalidate your score.
Training for Project Management Professional (PMP)®, PMI Agile Certified Practitioner (PMI-ACP)®, and Certified Associate in Project Management (CAPM)®