I didn't exclusively bf for long but like Erin said, I did find it hard to keep them latched well at the same time. Often you have to do the football hold and for me I just didn't like it very much and as awful as it sounds felt rather like a cow as opposed to a nurturing mother
like Ali said doing them separately does mean you can spend hours bf'ing, I remember one time I was sat at the same spot for hours with one off the other on b/c it took so long to feed them
once you all have the hang of it you can try 2 at the same time. Actually a way I liked doing both was to have them so that they were vertical to me cradled in each arm. This worked really nice at the hospital where you could set the bed and cushions to the perfect angle, but a bit harder at home, but it was pleasanter than having them on the pillow in football hold. However you're using both hands so if one comes off its a pain to get them to relatch. I would say be flexible and initially do whatever works for your survival