Since you are a Resident of Massachusetts, all of your income is taxable there, so in your example, yes, 90K is your taxable income in Massachusetts.
However, since you paid tax on 15K of that income to Rhode Island, MA gives you a credit against the tax you owe on your 90K for the tax paid to RI.
So your MA return should show 90K income, with the amount of MA tax you paid on your first W-2, plus the RI tax as a credit for 'tax paid to other states'.
Your RI Non-Resident return will show only the RI income and only the RI tax paid, which you may get refunded.
Here's more info on How to File a Non-Resident State Return.
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