Yes, cohosts and check-in staff can check in guests and scan tickets at the same time. Any number of devices can scan simultaneously. Checking in guests syncs in real time, so scanning the same QR code will show the guest as "Checked in", with an option to "Undo check in."
Check in access requires being added as a cohost or check-in staff on the event:
Note: Check-in staff have access to the guest list and ticket scanning, but they won't see full event details like the exact location address, and they're not added to the guest list or activity feed.