Is it safe to take zithromax and cephalexin a few days apart?

Antibiotics: "Safe" is a relative term. These are two classes of antibiotics; taking one should not directly have an adverse effect on the other. Taking antibiotics in general however has risks involved, some of which are due to alterations in your normal bacterial flora, promoting overgrowth syndromes like yeast infections, C. difficile colitis, and so on. The more antibiotics you take the more risks.

Yes: Follow your doctor's instructions regarding antibiotic therapy. Never save extra pills for later use.