Way to synchronize two NAS real-time?

We have QNAP NAS boxes on two sites that communicate via site-to-site VPN. I see a download of 2MBps on site #1 and upload of 2MBps on site#2.

The NAS have RTRR but it's really not fast enough to be able to keep up with the real-time requirements. Is there another way to have this real-time sync?