If you filed a T1013 form before, it should already contain the Reg ID, Group ID and/or Business Number you often use. To quickly prepare a 2nd T1013 form, you can do the following:
1. Open the first return file which contains T1013 in myTaxExpress
2. Choose menu "Start | Save As" to save the return file to a new name, maybe with new client's name
3. Now you are using this newly saved file, go to page 1 of T1, change client name and S.I.N.
4. Go to form T1013, it will be for the new client and has all the entries of Reg ID, Group ID and/or Buiness Number.
5. Old and new file co-exists, in this way, you can create more T1013 file.