How to calibrate dual subwoofers?

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.

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.

Myraj, get matching one won't help, but funny though.
Sean, welcome back. These are active subs.

Richingoth,

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. :> )