Move disks from Duo V2 to RN314

Unfortunately not. The units use different OSs and different file systems.
Best way to do it would be to buy new disks for the 314 and transfer the data from the duo to the new volume on the 314.
If you don't want to get new disks then you'll need another copy of all the data.
Do you have a backup of the data on the duo?
If so, make sure it's current and intact. Then you can insert the duo disks into the 314 and factory default.
Then copy the data from the backup onto the 314.
The latter method does leave you exposed with only a single copy of the data for a while though, I would not recommend it if the data is irreplaceable.

Thanks for the very detailed reply!
I think I will try to move all data to different PCs and then move the disks on the new NAS, it is the safest way...

No that's not the safest way unless you are proposing making 2 copies of all the data.
If you only have a single copy of the data at any particular time, then your data is vulnerable. You should always have at least 2 copies of the data, particularly if it is irreplaceable. You should also keep an offsite backup of either all the data or that which is most valuable/irreplaceable.

Well yes, you are right. It is the cheapest and simplest solution, not the safest.
I have to move 500GB of data, if the HD of the PC breaks down just when I copy them, it will be *very* unlucky!
What I can do is to copy the data on the PC, remove one disk and install it on the new NAS, copy the data on the new NAS, then move the second disk. In theory I should always have two copies of the files.
I've never added a disk when another one was in place: is it supposed to do all by itself or do I have to setup the RAID, then format, etc?

Adding the first disk is simple, just add it and then factory default the unit. The readynas will not use the disk as it is already formatted; unless you factory default with it in.
Adding the second disk can be done once you have your data copied over. You will need to delete any partitions on the second drive using a PC first. The 314 will not use the disk unless it is unformatted. Once you have done this you can just add the second disk while the 314 is on. Make sure the 314 is set to xraid, if that is what you want. Of you change from xraid to flex raid or vice versa, your data will be deleted.

To avoid opening the PC, would it be possible to put the first disk on the new unit, factory default, copy files; then remove the disk, insert the second one, factory default, then insert the first disk and let the NAS manage the copy with the RAID?

I am still not certain that the readynas will add the disk to the raid array if it has been previously formatted. Ŷou could try this, add 1 disk, factory default, copy data, power down, remove disk. Add 2nd disk, power on, factory default. Power down, remove disk 2, insert disk 1, power on, check volume is ok, then hot add disk 2.

It is easier to just delete the partitions with a PC- do you not have a usb:sata caddy or a USB:sata cable (one of the most useful tools I own.)

No, unfortunately I don't have the caddy...
I will see what I can do, maybe I can find one.
Thanks a lot for your precious help!

If you prefer I could help wipe the partitions table remotely while the disk is still in the ReadyNAS.

Thanks, very kind of you. I will try before the other solution and I will let you know!