Irelia Carries U told me about something like this trading combo of using Mark of the Assassin on the enemy and then waiting for it to be about off cooldown before ulting to the target for then being able to use Mark of the Assassin again after an auto for a second proc of Mark of the Assassin. IT was also easier to pull off this combo when Crescent Slash proced the secondary damage of Mark of the Assassin. There are videos of Crescent Slash being used to proc a second one but not seeing any of this "stacking" Mark of the Assassin.
However I have not heard about Mark of the Assassin being able to stack on a target but if it is able to work that would be an interesting "bug" along the lines of when riven could first easily wall hop that riot could expand on akali. (FYI she could wall hope in season 2 it was extremely hard to do and limited to something like 2 walls but I was shown it in season 3 before riven wall hoping was added with some changes in the movement system.)
A combo like this does 2 things to the enemy though. Allows for more damage in a trade should a trade happen, allows for some zoning of an enemy because no one wants to have akali proc the second part of Q expecially twice at the same time. add in a sheen or it's upgrade options and this could easily take 70% of a target s health.
Would also be nice to see this combo in action.