Do I need a TT jersey if I already have an aero road jersey?

It depends on your racing goals. An aero road jersey provides most of the aerodynamic benefit and is far more versatile. However, TT jerseys offer faster drying for swim transitions, are specifically patterned for the aerobar position, and eliminate pocket drag. If you are competitive in your age group and seeking marginal gains, a dedicated TT jersey is worthwhile. If you are a casual participant, your aero road jersey is sufficient.