How to calibrate dual subwoofers?


I have two subwoofers, Polk Audio PSW350 & Hsu VFT2. Each sub is placed next to each main speaker. I use the Y-splitter from the subwoofer output of the Yamaha AVR (used as pre/pro). I use the Radioshack SPL meter to calibrate. If I measure the subwoofers against the Left main speaker, there would be no way to tell which subwoofer is louder than other. When I watch movies, I could feel that the the left-hand-side sub louder than the right-hand-side sub. Does any one have any suggestion on how to make the left and right subwoofers to produce the same volume? Any advice or help will be greatly appreciated.
lej1447
Theaudiotweak,
What happen for most folks who have dual subs w/o variable phase controls, and I believe that is the predominant case? What is an alternative? Both of my subs have switches for 0 or 180, no variable phase control.
Use the same technique only switch between the two, 0 and 180..There will be a major difference, use the meter and a test disc to guide you..Phase differences are sometimes tricky to decipher..on movies and multi mic'd recordings..Tom
Thank you all very much. I'll definitely try both methods mentioned above. I'll let you know the results.
Thank you both Richingoth and Tom for your recommendations. I tried out your suggestions, and the room now sounds awesome. I place each sub next to each main speaker. Surprisingly, it sounds amazing. The mid range sounds clear during normal conversation. It does not sound boomy. The sounds get better with the help of dual sub during explosion scenes or rock music. Even though subwoofer produce non-directional bass, I could tell the improvement. It's weird. I like it, but it's weird. Not sure why. So weird.