I work international, and use one sim when in my own country but an other one when abroad (one because of the team offer from a cell company, the other cuz of the costs, of course). I used to change the sims at the border or in the plane, or to take another cell with me.
It would be much easier to have only one business phone with both of the cards build-in, so i can switch them by a button click - or have them both on at the same time.

Also a nice field of application would be the home/business phone separation. You could have both numbers one one device, switching them on and off by just one click.