Hi,
Based on my knowledge, the Read status syncs between different clients, so it's not feasible to keep the email as "unread" for user2 after the user1 reads the message.
I found this thread, in which the user asked the similar question:
https://community.office365.com/en-us/f/158/t/244520
You can check the workaround suggested by Henry Huang:
"Disable the “Mark items as read when viewed in the Reading Pane”
and the “Mark the item as read when the selection changes” settings.
By doing this, the user will need to manually open an email in Outlook client or set it as “read” by right clicking it. When checking the emails in the reading pane, the emails will not be changed to “unread” automatically. So that she can check the emails
of her boss and keep them as “unread”. But for her emails, a little bit more operation is needed."
Regards,
Melon Chen
TechNet Community Support
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