Im assuming Sarwan will take over the ODI captaincy after the World Cup and Lara will continue to captain in Test matches. Well that would be the logical thing to do anyway.
If Sarwan shows he is a good captain after about 1 year in the ODI arena, then he can take over from Lara in tests whilst Lara continues just as a test player if his ego allows.
This is exactly what happened when Vaughan took over the captaincy from Hussain for England. Vaughan proved himself in the ODIs first and then after this Hussain passed the captaincy to Vaughan in tests whilst still playing on. It was very successful.
Im still not sure about Sarwan as a captain, but seeing as though he has been identified and groomed as the vice captain to be the future captain, i think he has to get the first shot at it.