Dear Geric,
The sales order number field is the only one primary key (key field) in the sales order header table VBAK, and in system only one single record can exisit with primary key, so this is not possible to have the same number range starting with one only for both the documents, you can create different ranges like below
ZPOR - 1000000000 - 1999999999
ZSOR - 2000000000 - 2999999999
Regards,
Zafar