Warning regarding bathing two kids at once

So, last night I set the baby bath in the bottom of the shower and filled it up with water while undressing the baby and letting the pre-schooler start to play in the shower. 10 minutes later, the baby was happily bathing and kicking in her little tub (splashing a little water on me all the while). Just then, big sister pipes up and says, "That looks like water, but it's tinkle". I say, "no, it's not". She says again, "Yes it is, it's clear like water, but it's tinkle". After a few minutes of this exchange, I clue in to the fact that big sister did me and little sister a "favor" by doing #1 in little sister's tub before she got in it. Needless to say, big sister got a consequence for this action and little sister got a fresh bath.