If I lived in two states, do I put all of my military retirement income on each state tax form and let TT determine the prorated portion per state, or do I prorate it?

Did you change your state of residence during the year, or just stay temporarily in one or the other?

If you changed your state of residence in 2017 and have to file a return in each state, then you will allocate (i.e. prorate) the military retirement between the two states.  

Each state interview starts with your total federal income and then makes you allocate that income between the states you resided in. "Retirement income" is only taxed by your state of residence. You can travel and live in more than one state during the year, but only have to pay tax on retirement pay to your state (or states) of residence.

I receive a monthly military retirement check... It was just unclear whether I should put 100% of that year’s retirement income and let TurboTax determine a prorated portion automatically based on knowing the portion of the year I resided in that state, or if I should determine the prorated amount myself as 2017 retirement income on each of my state returns.

I understand. You will have to determine the prorated amounts and enter them in the income allocation interviews for each state. Just prorate based on where you resided when the monthly deposit posted to your account. You have to do the math since there are potentially many allocable items (sale of a residence, gain on sale of stock, property taxes, student loan interest, medical expenses, etc.) that don't necessarily align with your periods of residency. The program can't pick and choose how to allocate those items so they have you allocate everything.