The law does not require that you replace both your heating and air conditioning, but practically speaking you will probably have to replace both for the air conditioner to qualify.
Most central air conditioners (CACs) on the market today will only be able to qualify for the tax credit if you also replace the air moving device that pushes the cool air through the duct system (this "blower motor" is usually part of the furnace, but if you don't have a furnace it will be part of the air handler). Today's air conditioners need the added efficiency from a highly efficient blower motor (commonly an ECM, Electronically Commutated Motor, or another type of "advanced main air circulating fan") to reach the tax credit efficiency level.
If you recently replaced your furnace, check what kind of "blower motor" it has. If it has an advanced main circulating fan, then it's possible that a new air conditioner could qualify with this furnace.
Another factor to consider is that to get the required certification to prove your system qualifies for the tax credit, the equipment (both air conditioner and furnace) will need to be from the same manufacturer. This is because the systems (condenser, evaporator coil, and blower motor) have to be tested together and currently manufacturers are only testing and certifying their own equipment.
Read these FAQs for more information on heating and cooling systems that qualify for the tax credit: