I'm using Virtualbox with Vista as the host OS and XP as the guest OS. The only thing I haven't figured out is how to start the XP OS automatically when Vista restarts.
I have a HP slimline PC with 2GB of ram. I didn't try to make 2 calls at the same time. I did make calls from each line at different times.
I've moved on to plan B. Now I don't need 2 Magic Jacks on one PC. A friend tried a device from ooma.com that blows away the Magic Jack. With it I can port my current home phone number and set up a hunt to a second line if the first is busy.