First, 2 signatures was for the simplicity of the example, it is very low. We rather think of 8 signatures for our first try (~1 million members max).
You also have to see that there is a delay between each signature: during last FMM, we thought about 1 or 2 weeks between each. So the Sybils does not join instantly, we have time to see them coming. We could argue that this operation is time costly.
But more importantly, this type of attack can be lowered by using the distance rule. Typically, you just have to sign individuals you deeply know (family, neighboors, etc) and do not try to reach your maximum stock of signatures. This way, you limit the possible paths from you and maximize the distance between you and Sybils.
Finally, we have to understand that cheat have to be tolerated, by the simple fact we know we cannot avoid it. Even centralized systems cannot avoid it. So let’s assume this will exist. The only thing we want is to limit it so it won’t cause a currency collapse. By the way, it is exactly what our body does with bacterias 
And ultimately, this is a human organization. Other constraints may be applied on top of uCoin by them. Simply think about Laws.