Small separate desk for the 9-5 work. Can be in the same room as the home office space, but the sqft must not be counted towards the home office usage. Employer should be willing to provide compensation for use of your space and utilities, hopefully accountable basis, or enough to cover your costs and the increased tax (if treated as additional wages).
Or remove the sqft shared with the 9-5 use, and only claim the remaining amount (assuming there is some space not shared).
If you can document less home office use, you may have a number to show your employer as to your "cost" of the space you are providing.