Do one quality 15, rather than two crappy ones.
1. There will be less space taken up by a box
2. There will be less impedance rise, because there are less coils, so you will get more power out of your amp
3. One 15 with lots of excursion will displace just as much air as two 15's with lower excursion capabilities
4. There will be less weight
5. You can be proud of your brand, rather than getting some crappy Rockfords.
I think JL makes better subs than Rockford in general, and Rockford makes better amps. However, both are overpriced, imo. Would not buy anything from either brand.