User blog comment:Xilinoc/Ch. 682 - The Two Sided World End/@comment-3185063-20160716132236/@comment-1429792-20160717134022

Erm, yes they have met before. Yhwach went to visit him at the start of the arc. He had Loyd/Royd pose as him and fight Yamamoto while he broke in to Muken and tried (and failed) to convince Aizen to join him or some such.