Are you talking about hardmode/nightmare ops? Because our ops group haven't seen an enrage besides the one on Soa and even there it was because we slacked some time switching targets from mindtraps to the boss and back again. And we did all the bosses on normal with two tanks and dps that were in a very average gear to say the least - no set bonuses, no lvl 56 pve gear.
Also, on the pylons fight theres actually alot of damage to the tanks when two elites pop out. Probably it's doable if you CC one elite while killing another, but I dont see why you wouldn't bring a second tank on this encounter, especially on hard/nightmare. But as to the other bosses in EV - yeah, only one tank is needed.
Nightmare is incredibly tightly tuned for a few of the bosses (most very simple), Hardmode is meh-ish tuned enragewise.
Only real troubles you might be hitting enragewise are Hard/Nightmare Bonethrasher/Soa and possibly Nightmare Annihilator/Gharj if you have low DPS. Infernal Council Nightmare has an incredibly lax enrage timer and Pylons dont have one at all (or if they do I have not heard of it).
Bonethrasher can be carried on well into the enrage though even after hitting it so hes no biggy enragewise.