Hi, I've had the same problem, my wife and I bought one each for Christmas presents and both had this problem, as did the next two after exchange, the interesting thing is when we switched to white ones (others were all black), the problems mostly went away.
I have had the same problem a couple times since but my wife hasn't, the difference being that she carried hers in her purse where as I carry it in my pants pocket.
The faults occurred mostly when walking and sometimes when putting it down on a hard surface so I initially thought that it was due to the shock of walking/putting it down, but I was unable to replicate it by tapping the phone which would produce more g's than a step so I started experimenting.
What I found is that the phone, when flexed at the top ever so slightly will lose contact with the sim 1 slot, flex a little more and sim 2 goes as well.
Hopefully this will help the techs in replicating the fault and hopefully coming up with a solution to fix this as it is a major pain and stops me from recommending this otherwise great value phone to others.