Is there actually a pharmacological reason behind cocaine "ruining" the MDMA high or is it just that it overpowers and spoils the MDMA euphoria/empathy?
In general it is not wise to mix two stimulants, OP. Cocaine is a dopamine reuptake inhibitor and while MDMA releases minimal dopamine, there is still the potential for the two to cause dangerous overstimulation. While the risk is low if doses are kept to sensible levels, it is still present, and if you have a "decent" amlount of both then redosing might be an issue. I would definitely choose one or the other.
This is the right forum for combination queries

Ecstasy discussion takes place in the forum of that name, and cocaine discussion in here or Other Drugs. Please try to avoid using slang such as molly or blow - we're an international forum and it can be tricky to understand. I will edit your post, and I'll also edit your title to make it clear what you are asking.