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.
Sell one or the other and get a matching one, to the one remaining.
Are these passive or active subs? Sean
Myraj, get matching one won't help, but funny though.
Sean, welcome back. These are active subs.
Calibrate one at a time by disconnecting the other sub. Since you are running a dual mono signal the dB SPL should be -3dB lower than your L & R main SPL reference, ie. if your left main speaker is calibrated to the 75dB SPL reference than each of the subs should be set to 72dB SPL. (-3dB ref + -3dB ref = ref dB SPL)

Thank you for the advice. I'll try that. However, I have few questions for you before I try your suggestion. Since the subs are split from a Y connector in the back of the Yamaha, it will be tight and difficult for me to plug and unplug one another, assuming that I'll be content with one shot. Will it be all right if I keep the Y splitter on, just turn off one sub and leave the other sub on to calibrate, and do it vice versa. The mono signal split by the Y connector into dual subs should remain the same if I proceed with your method, correct? Again, thank you for all your help. You can tell by now that EE was not my major in college. :> )
You have to have a variable phase control on one...from 0 to 180 that is what you need..Measure the output of one channel including one sub.. say at 50 hz and record it, vary the phase and record it again ..At whatever phase setting you have the most out put that is where to leave it. Do the same for the other channel only check it and leave it alone ..Now turn both channels on play the same tone.. if they have complementry phase then you should have 3 to 6 db more output at 50 hz than when using only one..Having 2 identical subs will help the blend..Tom
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.