Like two-way loudspeakers, two-way earpieces use two drivers (bass & hi/mid frequencies) with a crossover circuit to split the signal into discrete bands. This generally results in greatly improved audio quality and more faithful dynamics.
As you might expect, the improvement in quality offered by two-way systems is particularly noticeable in the bass end, making them attractive to bass players. Apart from generally being very slightly larger, off-the-shelf two-way earpieces are physically very similar to their one-way counterparts, and are usually sold with a similar range of moulded fittings. The price tends to be roughly three or four times that of equivalent one-way systems, placing them roughly in the middle of the market.