When filing your tax returns as married filing separately, how do you split the joint brokerage accounts with interest and capital gains?

Example: we have a brokerage account with Fidelity acct xyz, which is a joint account with both our names on it. It has $1,000 of interest, $2,000 dividends and $5,000 longterm cap gaines. Is there a special form to split this, or does the IRS look at both returns to make sure each party submitted their half?