Review the item:
Latoya tried to concentrate on Professor Lemon's dull art history lecture,
but the darkened classroom and endless slide show soon had her dozing in
her seat, Latoya knew that Beverly would poke her with a
pencil if Professor Lemon bothered to glance
their way.
No punctuation is necessary between pencil and if. That Beverly would poke her with a pencil is one subordinate clause. If Professor Lemon bothered to glance their way is a second subordinate clause.
Subordinate clauses are incomplete thoughts and cannot stand alone as sentences. Generally, when you connect two subordinate clauses together, you need no punctuation.