League-of-legends – Peer to Peer transfer on LoL

Slow stacking

Slowing effects follow particular rules when stacking:

If a champion is affected by multiple slows, the strongest one will be fully applied, while the others are applied sequentially with 65% reduced effectiveness down to 35% of its original strength.

Slow effects derived from items' passives do NOT stack, even when coming from different champions. Only the strongest item slow is applied at any moment. The exception to this is the active effect of Randuin's Omen, Hextech Gunblade and Bilgewater Cutlass, which stacks with other slowing items normally because they are considered champion abilities for slow stacking purposes.

Attacking a champion with the Blessing of the Lizard Elder buff when the target is already affected by that slow will reapply the debuff, resetting its duration. It will NOT increase the slow amount. The same holds true for the Exhaust summoner spell.

Source

you could use exhaust on an exhausted champion, but the effects do not stack, the duration of exhaust might last a second or two longer depending on when the champion in question was exhausted. So it pretty much just gets wasted if not used sequentially right after another.