would a level 5 warlock/ level 7 wizard use the 12th level multi class spell slot table? PHB pg 164 under Spellcasting has your answer.
Under Spell Slots it tells you what classes you can add together and how to determine your total numbers of spells lots. Warlock isn’t one of them.
— Dan Dillon 👥 (@Dan_Dillon_1) November 19, 2019
