As they are both Phenothiazines and both act as Dopamine antagonists to varying extents with Prochlorperazine being a moderately strong potency anti-psychotic that is about twice as potent as Chlorpromazine while Promethazine is a very weak Dopamine antagonist with 1/10th the potency of chlorpromazine extrapyramidal side effects like TD, akathisia and other movement disorders is just 1 interaction between those 2. Another would be that they are both pretty potent anti-cholinergics so you run the risk of anti-cholinergic toxicity including delirium. They also both lower the seizure threshold so taking both together carries a bigger risk of seizures then either alone. So no they really can't safely be mixed together.
are you taking them for nausea? If you are taking both together won't really help. If you actually need these drugs for medical use id suggest going back to your doctor if they are not working as well as they should be.