Putting them together creates unique aggregates with unknown risks, essentially a frankenstein drug, which, at the least, will cause some to be wasted and do nothing, and the worst case scenario are unknown effects caused by triggering immune reactions or even potentially catastrophic problems that may not be apparent for years.
Some people don't sanitize vial tops, and reuse syringes.
Your risk tolerance is up to you, but don't make the mistake of thinking peptide/protein compounds are like AAS or other simple drugs that can be mixed without consequences,