That's also a good point. Interest is a percentage of the whole, if you don't keep its balance down even the cheaper card will run away from you.
I mean if you have two $1000 cards, one at 10 and one at 20% you'll want to pay off the 20% first. But if you owe 10% on 1000 and 20% on 250 which would you focus on? The first is costing you 100 per period while the second is costing you 50. Granted the second will compound more quickly and you'd need to do some nasty math that I have no idea how to do in order to figure out what it would really cost to pay off one versus the other over the long term... I think that's really the bottom line here, go find yourself an accountant if you really want to know specifics.
Ehhh, what? Are you suggesting that if you have 250 at 20%, and 1000 at 10%, you should pay off the 1000 first because it is costing you more per period? You're missing something. You pay off the 250, and... guess what... it's GONE. If you put that same 250 at the 1000, you still have 750 at 10%, meaning 75 per period PLUS 50 on the other card, equaling 125. What you proposed is costing you 25 more per month.
ALWAYS pay the most towards the higher interest rate card first, regardless of balance, and make minimums on everything else. Don't widdle both at the same time, as seeing the progress on the one card will be a motivating factor and it least set your mind at ease some about your debt, as you continue to pay it off.
OP, you're on the right track. As others have stated, if you have that much debt as a single individual, you'd serve yourself best by getting a second job while you are avoiding having fun (IE spending money) so you can get to debt free in a quicker time frame. Minimize expenses, if you have a really nice car that you can't really afford, sell it and buy something economical, and not brand spankin new. Best of luck.
EDIT: In addition, the rates you suggested (and I followed your example) are not exactly APRs. If they were, the interested rate would be extremely high. You aren't charged 10 or 20 percent per pay period. It all depends upon how often interest in compounded. In any case, you still pay off the higher interest rate card first, as it is generating more expense per dollar (read percent) than anything else you are paying off.