Since our inception in 2001 we have dealt with this very question "should i have 2 basins or 1 basin in the ensuite" and as we type this we are sure it will rage on for decades to come also. Is there a clear answer to this question for your bathroom design? No and here is why
Do i need this if resale is a factor, not in our opinion; this is not a luxury item space and design are the new luxury items in bathrooms so it is best to solve your immediate problems and the rest will take care of itself.
It is our belief that the most important thing is to have both washbasin / hand basin and bench space with a large mirror for two to share. Then if space allows for it put a second basin if it does not make the space feel small and cramp.