The siamangs and Francois langurs currently bookend the Discovery Center, but I'm sure you already knew they that. Though, how they think keeping the eight(?) Francois langurs in the same sized enclosure as two siamangs is beyond me.
In the wild siamangs live in small family units based on a pair-bonded male/female, while langurs live in larger troupe assemblages. Not saying the enclosures are sufficient or appropriate, just that the groupings are natural for the species concerned.
I visited the sifaka exhibit in September 2016 and it is wonderful. I'm also glad that the colobus monkeys were moved to one of the basic primate enclosures.