If you multiclass into two spell casting classes, what level of spells can you choose?

If you multiclassed into two different spell casting classes, would the level of spells you are allowed to choose depend on your total level of spell slots or the highest you could have from the specific class? I always though it was the former but Matt Mercer disagrees. https://www.youtube.com/watch?v=100wR825ImI&list=PLjJ7eKdrLY350toi0TdYGri-puOdyYCZH&index=27&t=0s

For example, if you were a wizard 3 cleric 2, would you be able to have level 1 or 2 wizard spells, and level 1 cleric spells, or up to level 3 of both.