Is there a way to set up a mapping, then a switch, then more mapping, with the second mapping cumula

Summary

Is there a way to set up a mapping, then a switch, then more mapping, with the second mapping cumulative on top of the first mapping?

Content

I have an integration with a lot of mapping.  90% of the mapping is the same for two types of records, so I want to do a mapping of 90% of the fields, then a switch based on a field value.  I need to map the remaining 10% based on the field value in the switch.  I want the second mapping to be cumulative on top of the first mapping.  I tested this but it appeared that the second mapping started from scratch.  Is there a way to make the second mapping cumulative?  Thanks.