Combined Driver via separate rule but results need to be additive

Summary

Combined Driver via separate rule but results need to be additive

Content

We have certain Cost Buckets which are allocated by Driver 1 via Rule 1. We have certain other Cost Buckets which are allocated by Driver 2 via Rule 2. Now we have a need to allocate a third category of Cost Buckets by both Driver 1 and 2 i.e. say 60% of values in that cost bucket by Driver 1 and 40% by Driver 2. How do we achieve this? If i try to manipulate the source cost pool of these buckets, for each rule, then the second rule would overwrite the results of first rule. Any ideas would be