One of you can deduct the interest and investments or you can split the amount between you based on the amounts that you each paid. You both cannot take the entire amount. For Married Filing Separate you need to prepare two separate returns, one for you & one for your spouse. To e-file, use your AGI from your prior year joint return, use the same amount for each of you, do not split it.
Generally, filing jointly will give you a bigger refund or less taxes due. When you file separately, your tax rate is higher and you won't be able to claim:
On top of that, if you live in the community property states of Arizona, California, Idaho, Louisiana, Nevada, New Mexico, Texas, Washington, or Wisconsin, you have to deal with community property allocations and adjustments, which adds extra work and complexity to your tax preparation chores.
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