Using the last of a vial

So I have like 5 units left in one vial and my total units are 60 so I’ll get 55 from my newer vial. How do you go about getting that last 5? Does this become two injections/two syringes? Do I take 5 from one use the same syringe and pull 55 from the other, Then inject with one needle? Would that mean I don’t push air into the newer one so I don’t mix them?