The XA20 has no internal memory, but it has two card slots. When your first card fills up, Relay Recording will automaticaly switch to the second card.
In the following link I learned some more about Relay Recording:
Relay Recording: Guide to Camcorder Relay Recording (http://camcorders.about.com/od/camcorders101/a/Guide-To-Relay-Recording.htm)
But information on the site of the Canon company tells me, the XA20 can also record in two partitions simultaniously.
What does that mean?

Storage media can be split into multiple virtual mount points or drives. For instance a hard drive might be partitioned into a c:/ drive and a d:/ while being a single physical device. I don't see the usefulness as it pertains to dual recording or relay recording, but it might be useful for storing one codec on one virtual drive and another codec on another virtual drive on the same media.

Jim is correct. The XA-20 can record in two different codecs simultaneously but then you can't use relay recording.

I currently have 128 GB cards in each slot so will run out of battery long before I worry about filling the cards up. While Canon makes a big deal about approved cards (meaning they tested them) these Komputer Bay Class 10 cards have worked just fine and were relatively cheap.

What then is the practicle use of recording in two different codecs? If you record only in the highest quality, you can make anything else in lower quality in post?

You have one version in lower bitrate mp4 ready for immediate uploading to the studio for rough cut editing or immediate streaming, and a higher bitrate version for final editing. Dual recording on this camera can only be done in mp4, not AVCHD.