Sivir at it Again: Block 2 spells at same time, this time, from SAME champion......

Though I've posted before on how she has blocked 2 or more spells from different champions, and received responses to how her shield must be procd and then activated thus being able to block one spell on the proc and the next one on the block (rare but possible situation since her shield ins't time-based like Fiora's) , I now do not understand how she can Step onto a Caitlyn Trap , block it, then block cait's piltover again within the same shield. I mean, this is aswesome for players that are shield masters, but if that's how it's going to work then I need to know the specifics so I can know when to try to block two spells.