Yessen,
Why are you not using SPNEGO on Java stack for Portal SSO ?
The SPNEGO on ABAP stack is part of NW SSO product, so is licensed. The SPNEGO on Java stack is not licensed.
If you don't want to pay for a licensed product, you can implement SPNEGO for ICF services such a the service desk application by redirecting to Java stack to authenticate the user, and setup the ABAP stack to trust the SSO2 ticket issued on Java stack.
Thanks
Tim