At MP10, you have the option of battling more than 2 uber boss at once. A player can take his infernal machine and craft to make one big single portal to battle more than 2 at once. The ubers that are behind the portal are random.
2 infernal: 3 Uber
3 infernal: 4 uber
and so on up to 6 at once.
Battle more than 2 at once have the following bonuses beside the 100% organ drop at MP10.
3 uber: 10% chance of a legendary item dropping
4 uber: 25% chance
5 uber: 40% chance
6 uber: 55% chance