Sasuke has tsukuyomi; yet he dosen't need it for susano'o.
First of all, the mangekyou sharingan has 3 abilities.
The third power is Susano'o.
Only people with two mangekyou eyes can activate susano'o.
As confirmed by madara. Any two eyes can activate susano'o, as long as they have mangekyou. Their true power.
So, the basic mangekyou sharingan has two abilities, then the special power.
The special abilties, kamui that allows you to warp space and time, and Kotoamatsukami that allows you to control somebody without them knowing.
Sasuke refers to susano'o as a third power. The left and right eye each hold a different ability. Kakashi's sucks in from a long range, obito from a short range. Following this logic, kotoamatsukami has another ability, though we will probably never know what it is.
Last, why sasuke has tsukuyomi.
Sasuke has all of itachi's eye abilities, plus EMS from his eyes. He dosen't need it for susano'o, he just needs both eyes.
Edit: Sasuke's special ability is controlling his amaterasu via blaze release, Itachi's is his tsukuyomi mastery.