I agree with others. They're both different sizes and can be used for different purposes so I don't think it'll be silly to have both. I have mezzo that I use mostly for school and I wouldn't mind having a piano as an everyday purse.
Initially I would say yes since they are virtually the same bag but different sizes... But if you like both and have a need for both sizes what's anyone else to say? I had the same type of situation when I had both a Cabas Alto and a Sac Shopping... I felt that they were too much alike so I got rid of my Sac Shopping... Big, huge, gigantic mistake! Now that I've re-acquired another Sac Shopping both are in my collection to stay! HTH.