Google Forms are not intended for respondents collaboration. The answers to each question will be saved until the user press the submit button. This will save all the answers that user had set and will not update the form that other user has opened before. To get the las answers, the user should open the form after the last changes were submitted.
If you need that several users work together filling different pages, instead of using a Google Form, consider to use a Google Docs document, spreadsheet or presentation. You or your respondents should make a copy or use publish the document as a template.
It's worth to say that the help article about collaborating with forms is for editing the form. From Share your form with collaborators - Docs Editors Help
If you're working on a form and would like to share it with a
collaborator, click the File and select Add collaborators.... Then,
from the "Sharing settings" dialog, you can specify individual
collaborators with whom you'd like to share your form for editing. To
add a collaborator, begin typing his or her name in the "Invite
people" text box.
When you're working with a collaborator on a form, each of you can
edit one input field at a time. For example, your collaborator can
edit the text of a question while you edit the help text of that same