Planning strategy group (20 + 21) for MTO+ETO

Hi

In strategy 20, MTO, requirement class is 40, with accounting assignment category M ,

requirements type is KE.

In strategy 21, ETO, requirement class is 80, with accounting assignment category D ,

requirements type is KP.

We have scenario in which second line item material of sales order or WBS (FERT only but say spares etc.) can either have strategy 20 or 21.

Can any one let me know, How to configure common strategy group, so that when I have sale order the material will take accounting assignment category M & when I have WBS the material will take accounting assignment category D?

Regards

Ojas Desai