how do i allocate my marketplace policy when filing married but separate

After you enter the 1095-A, you will see a screen asking if certain situations apply to you.   Choose the option I shared this policy with another taxpayer who is not on my taxes.   You can allocate the policy however you want, as long as the totals add up to 100%.

Where do I enter my 1095-A?

Please note - it is usually better to file Jointly if at all possible - Married, Filing Jointly vs Separately.   If you are married and you file your tax return using the filing status married filing separately, you will not be eligible for the Premium Tax credit (PTC) unless you are a victim of domestic abuse and spousal abandonment and can meet certain criteria.   Details regarding this relief are in the instructions for Form 8962 and Publication 974.