I had to add lemon juice (NOT RECOMMENDED- I was a poor junkie!) when trying that combination. It works, but you get pulp in your cotton (yuck). I had no success with just the two, which is what sent me to the fridge to find something acidic.
I found it better to get everything ready to smoke the rock, setting it nearby, and then taking the shot (if it wasn't of unknown purity which would require titrating the dose slowly, or a huge volume of liquid which might damage veins). I would try to hit the pipe before the heroin
Also, there wasn't the same 'synergy' as with a speedball, (if you can call it that), more like the two fighting it out for synaptic dominance. And I always got into trouble chasing the crack
comedown with heroin and then getting more coke
and then needing more heroin and then getting more coke....