How do I bridge 2 subs to 2 ohms?

If you are using a stereo amp that can push what they need, it's all good. That was the whole reasoning for making 2 ohm DVC and 4 ohm DVC subwoofers. To allow you the ability to configure your system how you want.

I know that Sony sold a setup similar to the Alpine package. but it was a stereo amp with 2 - 4 ohm 10" subwoofers. The amp was pushing a good amount of wattage for the subs in the package. There are many combinations for setting up a subwoofer system using mono, 2 channel, 4 channel amps. It's all about how you want to setup your system or how to setup a system with what you have. I don't think it is necessarily more common, but that would be for someone that does installations professionally to say. They would have a better aggregate base of data to determine what people typically setup for their systems.