Unfortunately, you can not combine scans from different projects into one project. If they still have access to the home: The best option is to go to open the project they want to keep and rescan the floor in that project.
Possible solutions:
If they aren't able to go back to the house and they have two separate projects, they can modify the infiltration area to match what is actually the total floor area of the home in each individual project. More about that here.
This will ensure that the infiltration calculations are more accurate, despite the entire home not being scanned as one project.