Release Strategy (Release Groups and Classes)

I created 2 Classes for release strategies which 1 is assigned Release Group 01 for the MRP run (for stock items), and the other to Release Group 02, which will be used for non-stock items. Class 1 have diffrent characteristics from Class 2, but wqhen i try to assign the 2 diifrent classes to diffrent Release Groups, it giovs me the follwing error message:

Only 1 release class should be used within the release groups for overall release and the release groups without overall relase in each case. However, in this instance several release classes have been used.

If i used all characteriscs in 1 release class and leave it blank in the release strategies (IMG), then the release strategy (ME51N) is not coming up.

the business is want to keep the characteristics as it is, and want me to work around it. The stock items strategy have been already implemented and operations and I want to implemnet it for non-stock items.

Thank you.