You are completely fine to do this. We're accessing these vials sometimes on a daily basis, so you're hardly increasing your contamination risk above the risk that already exists. Do the important basics, clean the top of your vials with alcohol, use bac water or bac NS to mix, etc.
Actually everytime you pierce the cap, add air from outside, you are increasing the risk. Unless you are combing them into a pen. If using syringes, its safer to keep the rest in their original vials until you use them.
No disagreement there about piercing a cap and really my point. Mixing a few vials together is hardly a no no in the risk world as we're almost all using multiuse vials every day. One extra poke for the mix is nada. Better advice to reduce the risk (which is very low anyway for subq injections) is, don't share or reuse needles, clean the skin, clean the bottle tops.
If they use that same vial to draw from each time they dose, its quite a lot more than one poke.
It's one poke to mix 2 vials. As for daily draws, a contaminated needle is a contaminated needle. It doesn't matter if it enters a vial accessed once or 50 times, the act is the contaminanted needle.
