Both GSM cards.
Yes, I was taken by surprise too when I saw my "gprs0" connection going crazy, I thought I was connected to a wifi by mistake, but no, it was the 3g. It was just one time(well, an entire week) and weird, weird because:
My carrier "gives" free low speed connection, and you can buy MBs in bigger speeds. Even buying it didn't work for me, so I gave up and used the low speed one, BUT, when I was in a Hospital (no, it did not connect to the hospital's wifi), that's close to an antenna, I started to receive all the MBs I've bought since I bought the sim card, all at once.

Well, I should have taken a snapshot, but now all that I have is one with th edataplan monitor showing 1gb downloaded via carrier data... not much appealing.

I'm sorry I cannot provide proof, just a testimony. I still have the 23th box and the 1gb snapshot, but it wouldn't prove much |:/