You can not double up the mortgage interest deduction on two returns. Meaning if the total mortgage interest paid in 2013 was $5,000 you can not claim $5,000 and your boyfriend claim the same $5,000 otherwise between the two of you you would be claiming $10,000 combined which would be more than what you actually paid. However, you can split the (in this example $5,000) interest between the two of you subject to rules below, meaning the splitting must make sense). If you "actually" paid for the expenses then you can claim what you paid. However, if your bf took the then he would have to amend back out what you are now attempting to claim.
Requirements to take the Deduction: If you are attempting to take a mortgage interest deduction then you must either be liable for the actual mortgage payment itself or have an ownership interest in the home. Merely paying the mortgage note is not enough to confer rights to take the deduction.
Ownership Interest: What constitutes an ownership interest in a home secured by a debt? Ownership interest can be either a legal interest in the property (such as having your name on the deed to the home) or an equitable interest on the property. Equitable interests arise when you are not the legal owner of the property; however, you put yourself in a position to take on the risks of ownership. This situation will be determined based on the facts and circumstances surrounding your situation. For example, supposed you and your spouse have bad credit and are unable to procure a loan to purchase your dream home. Your parents decide to help you and your spouse out by taking a loan out in their name and putting the deed to the home in their name, but allow you to live in the home from the time of acquisition. In addition, you and your spouse pay all of the home’s bills. In this example, you would most likely have an equitable interest in the property as you are taking on the risks of ownership by paying all the associated bills to the home, while the legal equity is being built up in your parent’s name.
http://www.journalofaccountancy.com/Issues/2008/Oct/EquitableOwnerEqualsDeduction.htm
Who can take the deduction? Once you have determined that you have a legal obligation to make payments on the mortgage note, or in the alternative have an ownership interest in the secured property, one must determine who gets to take the deduction. The basic rule is that you get to take the mortgage interest deduction if you satisfy the rule above and you are the one who actually paid the mortgage interest. Therefore, if multiple people satisfy the home ownership test and/or the legal obligation to pay on the mortgage test you cannot freely shift who gets to take the deduction. The person who actually paid the interest must take the deduction. While this may seem like common sense, more and more people are buying homes with their friends. Since only couples designated as able to file as “married” may file a joint return people may find that splitting the mortgage interest deduction results in itemized deductions, which are lower than their standard deduction. As such, they receive no tax benefit from the deduction and decide to give the entire mortgage interest deduction to one friend. This is technically wrong. Only the person who pays the expense, and only for the amount they actually paid gets the deduction (there are “gift” exceptions, but this is not discussed in this article. Also, a home which is owned by two friends and for which both are paying the mortgage is a situation where one owner cannot state they are gifting their interest payment to the other.).
Payment Methods: Payments made via joint accounts of co-owners are usually determined to be payments made 50/50. Method of calculating Deduction: If more than one person is eligible to take the deduction then you must both use a rational method for splitting the deduction. This is to prevent people from shifting all deductions to one party, especially if another party has issues with liens, debt, etc and is afraid that a refund might be used to offset other liabilities. Your method can be if the money comes from a joint account as 50/50 split; if the money comes from an account listing one person's name that the deduction goes to that person, etc. If you live in a community property state, then you need to follow the rules for community property division of income and expenses. Ironically, for mortgage interest the rules in community property on this matter is quite similar to non-community property states.
IRS.Gov Helpful link to deductions
http://www.irs.gov/Help-&-Resources/Tools-&-FAQs/FAQs-for-Individuals/Frequently-Asked-Tax-Questions...
IRS Publication 555 - Community Property http://www.irs.gov/pub/irs-pdf/p555.pdf
IRS Publication 936 – Home Mortgage Interest http://www.irs.gov/pub/irs-pdf/p936.pdf
