Okay... a few things:
1. What do you mean "it was me"? Could you elaborate?
2. Yes, I know it would be nothing romance related. This is the widowmaker as the main female role after all. It would be more of a bloody, twisted past-time both champions could enjoy.
3. Their meeting would most likely be as simple as crossing paths. Thresh picks out a target, one Evelynn has been hired to dispatch by a client, and, rather than fighting each other, they both torture the poor sap. Personally, I like to image Eve as part vampire, so she would take great interest in the fountain of blood as the target teeters between life and death, silently praying one of them slips up and ends his or her torment.
Only question would be who the victim is that could have political enemies who would hire the expensive assassin, and also catch the attention of the specter jailor.