You can split jointly owned savings and investments assigning percentage of ownership to each spouse and disclosing same in the explanation of changes field on the Form 1040-X.
You want to be clear on this as the brokerage likely has reported the earnings to the IRS under only one social security number.
Use language like "Joint ownership with X% reported here and Y% reported under so and so with social security number ###-##-####."
Your amended returns need to be filed by the May 17th filing deadline - after that date amending to changed from Married Filing Jointly to Married Filing Separately will no longer be allowed.
How to Change Your Tax Filing Status
