Don't mix them in the same vial or syringe.
It's a good idea if injecting them at the same time to use sites on opposite sides. Tesa is highly immunogenic, almost everyone experiences site reactions. You don't want another peptide in the same area the immune system is responding to.
Vaccines, proteins engineered to maximize immunogenicity, will sometimes use another compound or particulate that causes a strong immune reaction to induce a stronger response to the vaccine than would ordinarily happen. This is the opposite of what we want to occur with the peptides/proteins we don't want to build an immunity to.
BTW: these "immune stimulators" are called adjuvants. Particulate trash like glass and rubber particles in cheap Chinese vials act as adjuvants, as do aggregated proteins. Filtration removes these, lessening the overall immune response to peptides.
en.m.wikipedia.org