It will show as viewed whoever viewed it (primary or cc). If it’s viewed by both it will show as viewed twice.
Viewed Status Icon: If you open sales transactions (left navigation bar > transactions > sales) under status column, it will show many have opened the invoice and viewed it. It will show one black icon if viewed by one person or two black icons if viewed by both. See attached screenshot.
(Note: In case, you do not see Status column and need to add, go to middle Gear at the far right and select show more and add status column).
Invoice Activities: Open the invoice in question from the sales transactions or directly from customer center. Under activities (below invoice balance due, next in line to attachments), it will show invoice viewed with date and time record. But it is not possible to identify who actually viewed it (primary or cc person), especially if viewed by just one person unless one or both replied with the message. See attached screenshot.