How are deductions handled when married persons file separate returns? Most of the expenses come out of a joint account; must they be split in half?

Expenses paid out of a joint account are assumed to be equally split unless there is evidence presented that the actual ownership of the funds differs from the title on the account.

If you file married filing separately and one person itemizes, the other must also itemize, even if the first return filed was filed without itemizing. In that case, the person filing the first return must amend their return to also itemize deductions even if it is not to their benefit.