If you want to have Same SD&FI document number--then Extremely sorry to disagree with respected members.
Example:
Your Billing doc number range key: 01(numbers from 1 to 999999999)(Object:RV_BELEG)
Your Financial Doc type (example-RV)-with number key: Z1--(Numbers should be same like 1 to 999999999)(Object:RF_BELEG)
Now for your Number range assignments at OBA7(double click) or followed by FBN1--Activate the external indicator against Z1.
Now see --all your Billing document number and accounting document number will be same whenever any invoice/Cancellations done.
Note: If your requirement is having two billing documents/FI documents with same number--Sorry Obviously not possible..
Phanikumar