I would say that it's the same principle, but lacks the complete euphoric rush.
I have never co-administered a downer and an upper in the same shot, but I have injected amphetamine (which was part meth), cocaine, oxycodone and heroin.
Amphetamine offers next to no rush, just an immediate upper effect, with negligible euphoria. Oxycodone does the same - there is a slight rush of the high coming on fast, but no euphoric explosion emanating from your insides.
Cocaine IV and heroin IV produce the strongest, most euphoric chemical rush possible. I have a hard time believing that combining two drugs which do not benefit greatly from IV administration could match a combination of the two drugs that, for me, benefit the most from being mainlined compared to other ROAs.