Pre purchase planning

Check the hardware compatibility list for the RN102 and the RN312 here: http://kb.netgear.com/app/answers/detail/a_id/20641

If your drives are not listed, then Netgear will deny support if you have an issue.

Also note that the NAS will need to format the disks.

As long as you have two disks that are supported by the HCL, you can use them. They don't need to be identical though it usually is what people do.
By default the NAS will use raid1 and give you 1TB redundant storage. The storage can then be expanded by replacing the disks (on a 2-bay NAS you will get the smallest disk capacity available for storage, so you will want to replace both disks to expand the available storage). So with a 2-bay NAS you are limited to the highest disk capacity available for the NAS (or twice that amount if you don't use redundancy) and you are not stuck with the originally inserted disks.
If disks are formatted, you could factory default the NAS to get them fully recognised. I recommend a factory default after updating the NAS to the latest firmware anyway (to apply the 6.1.5 update improvements).

Mathias,
From my support days, I have not heard of any issues with using 2 x 1 Tera WD drives
that you describe.
You will need to Factory reset which will wipe them.

Install of ReadyNAS OS6
http://kb.netgear.com/app/answers/detail/a_id/7005

Can I move Duo/ NV+ /Stora disks into OS6 NAS?
http://kb.netgear.com/app/answers/detail/a_id/7007

Regards, Marto