First of all, combining the channels by definition means losing all the separation. Also do not try to combine outputs of amplifiers, it has a very bad effect on some. It has to do with DC and is insidious, sits there dissipating power uselessly. It can actually fry the amp in certain cases.

They make what you want for cars. Those ones that drive around "BOOM mufuka BOOM mufuka BOOM mufuka...". This is exactly what you want. It is active so it needs power, if 12 volts that is a simple matter because it does not pull much current. A spare wall wart in your junk drawer should do just fine.. Most of them also have a continuously variable crossover frequency as well. That comes in handy. Crossover too high and the subs might be too boomy, too low and the highs are taking too much power which they frequently can't use. Some have the option of combining the two channels, but at line level you can do it with two resistors and have practically no loss.

If you want it really loud, you might want a low pass passive filter on the woofers, such as in a regular crossover. This way if the bass amp clips it does not screw up the sound as much. But you probably won't need it. You are not really likely to clip anything with that power unless you have a huge outdoor party. In fact, with some exceptions people go to a party to mingle. For me, I like loud good sounding music, but not the whole time. People who just crank up the radio bug me. When I listen, everybody shuts up while it is playing and then there is an intermission between songs. Then there is time to talk, or maybe take requests, or whatever.

But seriously, go with the active (line level) crossover, you'll be glad you did. Most of them even have level controls, some power amps do not. Solves that, that is burns that bridge if you come to it.

There is a way to combine to a woofer on the output side but it loses a bunch of power and is totally unsuitable for bi-amping. Actually if you want to change your mind and go with four really full range channels, I can build you a little box that is like quad which is almost the same as Dolby surround.I mean native Dolby surround that they should never have been allowed to put their name on because it was so common in the 1970s+. I used the technique for movies with my Advent five foot TV, which BTW was the only one in town with really hot tubes and near perfect geometry and convergence. But like on Days of Thunder you heard the cards actually go around you when the camera angle was that way. The Enterprise would take off and seem to go above you on the screen and the speakers sounded like it. It is good for some music, but not all. Some is mixed funny, like all the bass in one channel, like The Swinging Medallions - Double Shot Of My Bsby's Love or Beatles - Taxman. Those are not suited well for such a system.

It sounds like Dolby without any delay or anything like that. In that respect it is true high fidelity because it only puts out what goes in. With all the effects, Dolby surround is not high fidelity, it *******ises the sound with.

Actually with speaker that have really good imaging it almost sounds like there are rear speakers. But you'll probably not find them with that kind of power handling without mucho dinero, and very low efficiency.

So really, just go to one of those car audio places and tell them what you want, most likely they have it on the shelf already. You don't need an ultra high fidelity one,just low distortion, and very importantly, LOW NOISE. While the levels are line level, you are dealing with it after the volume control. So any noise introduced by the unit will still be there.