I moved from MA to MD in April 2010. I made a steady wage and my MA W2 taxes went to MA and my MD W2 taxes went to MD. I prorated my bank interest correspondly (essentially 1/4 MA, 3/4 MD, but it was aligned with the exact specifications and calculations).

However, here's where the issue is. Previously, I had carry-over loss in MA and applied the carryover loss to all of the dividends (e.g. $1000 dividends with a carry-over loss of $2500 results in $0 dividends and $1000 carryover loss applied to dividends, $1500 remainder). Now, in MA, you apply carryover loss to dividends *Before* any prorating of residence in MA. Therefore, I'm essentially paying 100% of my dividends to MA. This is all fine and dandy in MA, but makes it difficult when in MD because I do not want to be doubly taxed on my dividends. So I'm wondering if I adjust for this in MD Form 502 Line 5, Line 12, or Line 27?

Line 5. Other additions (code [A] and write -$1000)
Line 12. Income received during period of non-residence (write +$1000)
Line 27. ? Using MD 502CR?

“ This is all fine and dandy in MA, but makes it difficult when in MD because I do not want to be doubly taxed on my dividends. So I'm wondering if I adjust for this in MD Form 502 Line 5, Line 12, or Line 27?”---->You are a part year resident of MA and a part year resident of MD as you moved from MA to MD in April , 2010 as you said; Imean you have no home state temporarily at his time, as you can see; as there is no reciprocal state agreement between MA and MD. so, as a part year resident of MA, you need to file your MA state return on your MA income between Jan 1 2010-April , 2010, and you also need to file your MD return as a part year resident of MD on your income earned in MD probably between April, 2010 – Dec. 31 , 2010. So, you can’t claim your taxes paid to MA state on your MD return and vice versa. You can claim state tax credits paid to both MA and MD on your federal return. You are NOT subject to double taxes in MA and MD. As long as you have your dividend income in MA, then it will be subject to MA state tax .If you still have carryover loss after you become a full year resident of MD, then you can apply it on your MD state tax return.

Yes, MA & MD don't have reciprocal. However, according to the instructions, if I lived in MD for more than 183 days, I am a statutory resident of MD and non-resident/part-year of MA. All my W2 income and interest is accounted for properly. So now I'm still confused where to deduct the dividend from MD taxes (Line 5, Line 12, or Line 27).

Are you sure I can apply a carryover loss to MD tax return? According to another guy from Intuit, I can't. I mean, there's no record of how to transfer carryover loss from MA to MD, both for MA and MD tax forms. That is, if I transferred carryover loss from MA Sch D (e.g. $1500), there wouldn't be a place on MA to deduct. So if I applied carryover loss to another state, the loss would still exist in MA (Sch D) essentially making money out of nowhere, correct? MD always immediately applies carryover loss since it piggy-backs off the Federal so there would be a spot to apply this carryover loss too. So I don't think I can apply it.

“if I lived in MD for more than 183 days, I am a statutory resident of MD and non-resident/part-year of MA.”-So you were physically present in MD for more than six months in 2010, then you are a full year resident of MD. “If you were domiciled in Maryland on the last day of the taxable year, or you maintained a place of abode (a place to live) in Maryland, and were physically present in Maryland for more than six months of the tax year, then you are a legal resident.” You are a still part year resident of MA. Then I guess you, as a full year resident of MD, need to file your MA return as a part year resident of MA on your MA income between Jan 1 2010-Apr. 2010 and you can claim your MA tax paid to MA on your MD return.
Maryland Income Tax Information - Residency Status
“Are you sure I can apply a carryover loss to MD tax return?”---->As you said, as a full year resident of MD, you can apply the carryover loss to your MD return.
“According to another guy from Intuit, I can't. I mean, there's no record of how to transfer carryover loss from MA to MD, both for MA and MD tax forms. That is, if I transferred carryover loss from MA Sch D (e.g. $1500), there wouldn't be a place on MA to deduct. So if I applied carryover loss to another state, the loss would still exist in MA (Sch D) essentially making money out of nowhere, correct?”----> As said previously, I guess so as a full year resident of MD because you can never apply the carryover loss in MA any longer after 2010; you are a nonresidnet of MMA afteer 2010. You MAY need to contact DOR of MD or MA.

“ MD always immediately applies carryover loss since it piggy-backs off the Federal so there would be a spot to apply this carryover loss too. So I don't think I can apply it.”---> Not sure.