Curb appeal is just as important for duplexes and apartment buildings as it is for the single-family home. This is even more critical with multifamily because a messy yard disturbs not only neighbors but the other tenants of the property. Toys, bikes, or out-of-season tires stored in the yard are an eye sore for anyone. If a landlord allows one tenant to store items in common areas, it is only a matter of time before others follow their lead.
If a garage is available, hire a contractor to bump out a small locker for each unit or do it yourself. Hardware stores often sell stand-alone sheds starting in the low $300's. If your tenants still have too much stuff, direct them to a local storage facility.
Be sure to add a clause in all leases prohibiting the overnight storage of items in the yard without prior permission from you.