With high sample rates (in the GSPS realm) also come a huge payload of data (bits per second). In order to transfer this huge payload, the JESD204B high speed data transmission protocol was adopted. The JESD204B is a high speed, data transmission protocol that employs 8b/10b encoding and scrambling among other features aimed at providing adequate signal integrity.
With the JESD204B comes a big alphabet soup which at the outset sounds utterly complicated (which in a way it is). The addition of digital processing blocks such as digital down converters also adds to this confusion. In order to mitigate this confusion and to make the implementation easier to understand, the concept of virtual converters has been introduced.
Read about it here : Presto! Double Your Number of ADC Channels | Analog Devices
So, when all of a sudden you see "four" virtual converters on a dual-channel ADC, you now know why....