The cocaine's effects would have probably reached their zenith—and soon afterward, their nadir—around the time the PCP kicks in. As a result, I'd presume it's unlikely that the drugs' disparate onsets would provide adequate time to allow for the possibility of some type of disastrous and regrettable interface or synergy occurring.
The effects of intranasal cocaine have an onset of around 5-10 minutes and a duration of roughly 25-40 minutes. On the other hand, intranasal PCP—which is rather anomalous, as most PCP is liquid or added to weed, mint leaves, tobacco, etc—can be felt somewhere around 30-50 minutes after administration and lasts for up to 3-4 hours.
As can be easily inferred from these onsets and durations, their isn't much interaction that could possibly take place in that time interval, as the cocaine would be just about finished around the time the PCP begins. You'd be better off starting with a rail of PCP first and then the cocaine about 25 minutes afterward.
But I don't understand why you have to do them both. PCP is already fairly stimulating, as far as dissociatives go. Too much stimulation and mobility would make me nervous. That's when you're prone to embarrass yourself or get into trouble.
