You can split the expenses any way that you want as long as the total amount is not exceeded between the two tax returns.
Since you are not married one of you can use itemized deductions and the other use the Standard Deduction, whichever is more beneficial.
NOTE -
The total of all your itemized deductions on Schedule A must be greater than the standard deduction for your filing status to have any tax benefit.
Standard deductions for 2023
Single - $13,850 add $1,850 if age 65 or older
Married Filing Separately - $13,850 add $1,500 if age 65 or older
Married Filing Jointly - $27,700 add $1,500 for each spouse age 65 or older
Head of Household - $20,800 add $1,850 if age 65 or older
