Can I take promethazine with hydroxyzine?

No, it will widen your QRS complex and you will 'brady out'.

In english, it will slow your heart down and may cause it to stop, therefore

you will die in your sleep as it also has a sedative effect. So, I guess, if you want

to die, there you go, I'm not sure of any uncomfortable or traumatic side effects (you may wake up unable to breathe), but personally I wouldn't take it, apart from not wanting to exit life at this point, I don't like the idea of knowing I may die as I drift off to sleep....