I guess the best way to put it is that it'd be supplemental. The design I had in mind for that would be Mix as a lv30 ability on a 30s cooldown, with 3 chemicals being usable while Mix is active in the 30-50 range (HP restore, regen, damage shield) and gameplay would involve combining these effects. Then we add a second line of potions to combine with these effects in the 51-70 range. While the effects might be similar, the main appeal is that some create specific effects when used second. One example is Chanterelle Extract (restores HP) and Diluted Treant Sap (restores HP). Both restore HP with a cure potency of 200, but if you use the Sap second you get a reaction that restores additional HP with a cure potency of 400; your total healed from this mix would be 800 potency. Yes, this is OP for an ability on a 30s cooldown, but the numbers can be tweaked.
Another example is Regrowth Formula Alpha (100 potency regen for 15s) and Regrowth Formula Beta (applies Chemical Residue). Alpha is technically "better" because its regen has higher potency per tick than Chemical Residue (80 cure potency), but if you use Beta second you get the reaction of applying an additional stack of Chemical Residue on all affected targets if they are already affected by Chemical Residue (meaning you'd want to do something like Airborne Medicine => Mix => Regrowth Formula Alpha => Regrowth Formula Beta => stacked HoTs).
This means that, in theory, you could choose which chemicals to combine at higher levels depending on what's needed. You can use the regrowth formulas if you want to stack HoTs. You could also use Regrowth Alpha followed by Treant Sap to have a 15s HoT and a 600 potency heal.
I mean, you're not gonna get things that increase damage or decrease damage taken (Pluto Concentrate and Stimulant exist, after all), but I feel it could prove useful since it'd be a boost to healing output without having to burn cooldowns for it.