TVS unidirectional vs bidirectional in Hbridge
Another aspect, should I add 2 more TVSs across the high side MOSFETs
  also?


It could be argued that the only TVS needed is the one between 24V and ground. The diodes inside the MOSFETs will steer any back-emf from the motor to the rails - think of these four diodes as a bridge rectifier (because that is exactly what they are) and think of the motor as the secondary of a transformer. The output of the bridge clamps the voltage thus protecting the MOSFETs (which happen to be across the four diodes).

The back-emf from the motor will be limited to the current in the motor and so providing the diodes are rated sufficiently above at the maximum current that can be taken by the motor there should be no tears.