Have you calibrated your current set-up? If not, I would suggest you do that first before considering whether to add another sub (and it's cheaper).
Adding a second sub can be a good thing, but with some caveats. One is that you're dealing with good subs in the first place (I'm not familiar with the specific models you described). A single good performing sub (and sized adequately for your room) is going outperform two lesser subs. Combining subs can help smooth out the frequency response in your room, but that typically entails calibrating both subs - which can be very difficult with two different subs, vs. two identical subs. Based only on your description of the subs (nice Infinity sub, cheap Polk sub), if you're set on buying another sub, I would consider another Infinity first (assuming you've done the calibration on your current set-up and know you've got issues that might be solved with another sub). Hope that helps.