Yes, it is possible to define the seismic weight/load two times in one STAAD file. For this purpose you will need to use two different seismic load cases. After you define the first case, simply go back and add the second one using the parameters you want. After that, you can run the simulation. Check the official forum for further information. Also, if you have issues, use the contact information from the official web page to get in touch with the support team.