Is better to have a mono amp or 2 channel amp to 2 sub-woofers?

You won't notice any stereo separation with two subwoofers because our ears perceive bass as omnidirectional. Two subs can make more volume than one of the same size. If you are using two subs, you should either use two separate mono amps, or a two channel setup. As long as the amps have a high pass filter they will work equally well, although the wiring will be simpler for the two Chanel setup.