Hi Sai
Kavita is 100% correct.. Doc type SA is not meant for splitting.. I had a similar requirement as yours way back in 2007/Sep...
I raised an OSS message and SAP confirmed that SA is not meant for Splitting
As Kavita said, creating a new doc type is the solution..
You can validate yourself.. Just try it with new doc type as mentioned above and you can know the result yourself
Br. Ajay M