AVRs/AMPs and Dual Subwoofers

Dual subwoofers can be a real benefit to any home cinema or music system when integrated properly.

Positioning is the first and most important part.
The Benefits of Dual Subwoofers
A Guide to Positioning Dual Subwoofers

The second aspect is the correct calibration. 
Most AVRs and integrated amplifiers only have one subwoofer channel that can be calibrated, even if they might have two subwoofer outputs.
It is crucial to make sure you can calibrate two subwoofers individually, as you will always have variances, due to room dimensions, furniture, and other placement compromises. The placement will never be 100% symmetrical.
So please check the manual before buying a specific AVR/AMP that it supports the features you need.

If you already own an AVR or integrated amp that only supports one channel, you can always add a stand-alone DSP like the very popular miniDSP 2x4 HD. It can correct for up to 4 individual channels and works well with the commonly used measuring software REW.