No absolutely do not mix peptides in the same container, syringe or otherwise.
All you are doing is is creating more points for peptides to adhere to each other, forming unique aggregates, effectively creating millions of random new drug molecules, which, at the least will inactivate a certain amount of the peptides and trigger more of an immune reponse.
The FDA has warned about this reckless practice with unknown consequences. There are no pharma drugs that include multiple peptides. The few that have considered combining two, none of which are approved, used polymers to ensure they couldn't stick to each other.
People need to get over the notion that if they inject something and they don't feel anything bad happen within a day it must be fine.