Parallel structures need to be the same, whether they are phrases, word combinations, or clauses. To make this sentence a parallel structure, you need to put the two independent clauses into the same format. The first clause is "No two snowflakes are exactly alike" and the second one (connected by the conjunction "and") is "leaves are all different, too. So, to combine them in a parallel way, there are a couple of possibilities:
Neither snowflakes nor leaves are exactly alike.
Snowflakes and leaves are all different.
No two snowflakes are exactly alike, and neither are leaves.
No two snowflakes and no two leaves are exactly alike.
Or, change it up entirely: Snowflakes and leaves have very unique structures.
Take your pick!