It all depends on dosages.
If you already know how much to take of both of them, without being overwhelmed in singularity, you can combine them safely.
But if you OD on one or the other, then you will either have a heart attack(from speed)or, respiratory arrest(from the heroin.) So make sure you absolutely know the right balance beforehand.
If you take a barely get high dose of them both, then you are good.
The main mistake most snowballing people make is with potency variations between heroin bags. It's almost suicide to do a snowball with shit from new bags you just bought, as you cannot properly gauge a correct dose right off the bat, leading to a fatal mishap of measurement.