Canon 90 EX for beginner as a Master unit?

Hi, I do have experience of using 'Dual' TTL cords. Canon do not make one - their OC-E3 has a single female hot-shoe.

'Dual' TTL cords can confuse the camera. They are OK under ETTL if only one attached Speedlite is operating in ETTL - for max reliability, have only one female hot-shoe occupied (& that attached Speedlite can be enabled as a 'master/controller' device, if you so choose).

TTL cords from Syl or Flashzebra are of the single female hot-shoe type only (at least, when I last looked).

Good luck. Donald