EX2 Ultra systems, trying to create remote backup over internet to attached USBs

OK. here’s what I’m trying to set up. I have an EX2 Ultra, with a USB drive attached. I back up to the EX2’s internal discs. My sister has the same setup on her network. We now want to protect against burglary/fire/flood, by saving our data to each other’s houses. Idea is that she logs onto my EX2 across the internet and uses her backs up to its attached USB (ie not to the internal drives of my EX2 because they’re full of my stuff). And I want to do the opposite - back up to the USB-connected drive on her EX2, not its internal drives.
Is this actually possible?
At the moment I can log on to her EX2 via a browser using mycloud.com and her login details, and I can see the USB. But I can’t manage to start a backup job via the dashboard. I just get an RSYNC error, whatever that is. All help greatly appreciated.

You can use the Remote Backup Feature to backup the data from your My Cloud EX2 Ultra device to other My Cloud EX2 Ultra device setup on different network. The data under remote backup will be backed up initially in the WD My Cloud EX2 Ultra set up on destination network. You can refer the link mentioned below to know the steps required for Remote Backup set up.

Once the remote backup is done on the My Cloud device, you can setup the NAS to USB backup job from the Destination’s My Cloud device to backup the data in the External USB device.

Thanks. But before I bought the EX2 I specifically checked with WD support that I could back up to the remote USB without my data going into the remote EX2. I don’t want to fill up her EX2. I just want to fill my USB disk attached to her EX2.
In the opposite direction she has now managed to do this, backing up her data to a USB disk attached to my EX2. But going the other way I get a series of ‘failed RSYNC test’ messages. I’ve gone through all those support articles sadly.