Two years ago a buddy and I spent 10 days hunting in Zimbabwe. We were going to go 1x1, but 8 days into the hunt, one of the PH's had a family issue come up, and had to leave, so we spent the last two days 2x1.
If you're even remotely competitive, as we were (and are), then I firmly believe 1x1 works best. Having said that, by day 7 we both had our buffs, and most of the other "headlline" game we were after. So no pressure, and little to compete on. And I have to say, we had a great time, taking turns shooting. The thrill of getting the animal is greater if you have a friend (or family member) to share it with.
I think it would be a mistake, though, to undertake any of the specialized hunts - cats or elephant for example - 2x1, but it's a great way to maximize the fun factor if there's not too much pressure. But make sure you set the ground rules in advance - does "my turn" mean shooting an animal or shooting at an animal? Do we stop for (and maybe spend hours following up) your second impala when I really want my first kudu?
So overall, I can't say one is better than the other. It depends on the circumstances. 1x1 is almost always going to be better for getting the game you want, while 2x1 is almost always going to be more fun, if you're with someone you enjoy hunting with. Decide what matters most to you, and do that. It takes a pretty jaded person to complain about any hunting in Africa, whichever way you go!