Both the above are right - if you nasally take a *safe* amount of each you wont die, hell, with ahealthy heart you could probably IV it and get away - I admit to having done it - recently, and it seems I have people comming at me left and righttelling me how they also did it with good success...
STILL, I DO NOT ADVOCATE SPEEDBALLING BECAUSE THERE IS SUCH A HIGH RISK!... then again, if you stick to SNORTING these you should be cool ... just start small - you can always do more...
Here is something else to consider, and THIS IS IMPORTANT - The cocaine is goingto wear off a LOT FASTER THEN THE HEROIN, so, at first you might rail a gagger of heroin, do some coke, and figure "hey, I can do more smack, I'm speeding so hard I coukld use the downer" ... wel, 20 minutes later the coke high is just about gone and you now have to contend with all the heroin you did!
It can be fun - but please be carefull!