i don't remember diazepam or any other benzodiazepine having a direct effect on hearth rate, the reaction you mentioned could be an isolated case or maybe because some other drug or pre-existing condition affected their hearth rate. if anything it should actually have the opposite effect.
fentanyl by itself does the job just fine, specially if you have low or no tolerance. combining it with any benzodiazepine is kind of redundant because of the potency of F, but if you want to use them both then go for it, they actually have some interaction because both are downers and when combined they usually cause respiratory depression, in fact many fatal overdoses of heroin include use of benzodiazepines.