Differing implementations for a function in two interfaces

[Ryan Steen]

In article <e1vlnb$226i$1 at digitaldaemon.com>, Lionello Lunesu says...
>IA might be an interface for the 
>output/GUI and IB for the input.

Yes, but what might cause the need to have the implementation in one class?

And if there is really a need what hinders a virtual split of that implenting
class?

IA --> CA <-- C --> CB <-- IB

where --> and <-- denote inheretance?