Example of a moulded plastic dual RCD consumer unit as commonly available in 2015.
This type of plastic unit is no longer permitted, most newer types are steel. However the devices and internal layout are identical.
Dual RCD consumer units are a cheaper economy option, the disadvantage being that a fault on one circuit can affect several others. In the example above, each RCD covers three circuits so a L-E or N-E fault will cause three circuits to be disconnected.
A far better option is individual RCBOs for each circuit - combined circuit breakers and RCDs. With these, any fault will only affect one circuit. They do cost more than the dual RCD versions, significantly more with larger numbers of circuits.