Paying down 1 or 2 morgages faster with extra payments

Hi all,

I have two recent mortgages both at $175k principal and 6.625% fixed for 30 years. I want to accelerate the payoff by either paying $500 extra on each or $1000 extra per month on 1 only and keeping the regular payment for the other. I ran some numbers and look like paying $500 extra on each is better with lower total amount and interest paid. Also, paying $500 extra reduce the payout period by about 15 years. At $1000 extra there is additional 5 years reduction only. I wanted to check if others have came to the same conclusions? Also, is there a nifty tool to calculate incremental return of additional principal payment? If yes, then it will be helpful to decide how to optimize additional payments across different mortgages. Any other tip for this newbie in the REI world? Thanks.