That's right, with the level cap restriction removed off CoP 6-4(Ultima/Omega) it is now possible to duo it quite easily.
I 2boxed it as 75 MNK/NIN + 75 RDM/WHM for my RDM char last night. Fairly basic strategy, no special preparation, used 1 CCB Polymer but really didn't need to.
On entry, RDM self-buffed(haste, refresh, pro, shell, phalanx, SS, aquaveil) and popped a taco, then buffed MNK with the standard haste/pro/shell. Red Mage supertanked the Mammets while MNK peeled them off 1 by 1. Mammets went down in 5 minutes, approximately.
For omega, RDM self buffed nondefensively(haste, refresh, that's about it) and gave the MNK Haste as well. MNK blinktanked Omega, RDM fully debuffed it with Slow 2, Para 2, Dia 3, and blind(all stuck without issue). Cured for first 75% or so, then nuked it down.
For ultima it was the same strategy as Omega except the nuking began in earnest at 70% instead because I wasn't concerned about burning convert early.
Finished the fight in about 25 minutes total, including the 10-15 minutes of cutscenes and whatnot