When rocket scientists say "any two" they usually mean it. It can probably tolerate more, if they fail just right, but you can't assume that.

And it seems plausible. The layout of the SuperDraco clusters is not equidistant; it's more of an "X". If you lose one cluster, the other nearby one can provide the makeup thrust without being too unbalanced, and there's still X and Y control authority from the other pair.

Watching the video again, I see that the SuperDracos are mounted pretty high up in the sides. Given the fuel tanks, Draco manouvering thrusters, life support system and so on are mounted lower down or in the base, plus the weight of the heat shield, I suspect the SuperDraco's are mounted above the centre of mass of the Dragon. This means it would be statically stable on just two opposite clusters. Four SuperDracos have 64,000lbs of thrust, which should be plenty. So, yes, I think it could cope with losing two engines in the same cluster.

Excellent question. I was wondering the same thing.

It seems like it must be two engines that aren't in the same pair. If two engines in the same pair went, you'd have to cut off the opposite two to maintain symmetry, and then you're down four. If it could handle losing four engines, you'd think he would have said that....

On the other hand, maybe they can lose four engines as long as their opposite each other, and he didn't say that because it's not any four. It's not completely implausible. If each engine runs at 50% normally, then increasing the remaining four to 100% would give you the same thrust. Attitude control would be the big challenge, but perhaps smaller thrusters are able to handle it.

Edit: I just realized that the landing thrusters double as the launch escape system. The LES needs really high thrust to get the fragile meaty cargo as far away from the exploding rocket as quickly as possible. The Soyuz LES produces accelerations of well over 10 gees, for example. If these engines are capable of that, then they must have huge margins, as far as thrust goes, for landing. Looking over reports, I must have missed it in the stream, but Musk said that each one generates 16,000lbs of thrust. While the capsule as a whole weighs well under that... and it has eight engines. It also looks like the engine pairs are angled apart somewhat, which should allow for some control even on only two opposite pairs. I'm going to go ahead and guess that it can tolerate losing any two engines, and up to four if it's lucky about which ones it loses.