You could do something like have a book for learned potions or something? Maybe have a description of the potion and her thoughts on it and a button to trigger the scene immediately with a section at the bottom about combined effects? Like after unlocking the tentacle or futa potion, have the bottom say "Combined Effects: [?] [?] [?]" Where each question box is another option that would unlock when she learns the potion. By clicking on that option it would show a scene of her finishing mixing or drinking the two potions at once followed by the combined potion effect.
This would also make it so you could choose how many combo scenes you want for each potion without having to figure out what to do if a potion is saved and doesn't have a combo scene for the next one or make space to hold all previously learned potions.
It could be some fun story building as well, if you have her add her thoughts either under the effects of the potions or after. I would want to know her reactions after the futa and bimbo potions personally.
