The voltage ripple on the output is the integral of current in the output capacitor(s). If we assume the load draws a constant current then for the ripple voltage to be zero the current delivered from the switch mode converter(s) must also be constant.
If the ripple current from the switchers was a nice clean sinusiod at the switching frequency then what you propose would be possible. Instead as the name suggests it's a waveform resulting from switching, sometimes the current may be zero (bucks running in "continuous mode" do deliver current all the time, boosts, flybacks and converters running in discontinous mode don't) and when current is being delivered it's not quite at a constant rate (how close to constant it is depends on the value of the inductor). The exact shape of the waveform will depend on many factors including supply voltage and load current.
Having said that while your method won't completely eliminate ripple it will significantly reduce it. We call this a "polyphase" converter and it can be done with any number of phases from 2 upwards.
For now lets assume that the waveforms from all the converters are time-shifted copies of each other and are periodic at the switching frequency (these are approximatations of relality but good enough for now). We can view any periodic waveform as a series of sinusiods at harmmonics of the switching frequency.
If we have n perfectly matched phases then we eliminate any harmonics that are not a multiple of n. In general lower harmonics tend to be larger than higher ones for most waveforms and on top of this the output capacitor is acting as an integrator which means high frequency ripple currents have much less impact on ripple voltage than low frequency ones.
This is done in practice, for example http://www.linear.com/docs/4166 describes a converter IC that is designed to drive two phases directly and provides functionality for syncronising multiple chips to build converters with up to 12 phases.