bridging a 2 channel amp to 4 channels

First excuse me if I aint writing the subject correctly.

My friend told me that I can run a 2 channel amp to power my sub and my 4 other speakers. I told him that I already asked that here and that I was told that it was possible but that I would only hear one channel out of the 4 speakers. So if the sound went from Left to Right, I would not hear anything on the 4 speakers. He said that there is a way to bridge the amp to power the sub and still get the 4 speakers to play as if it were a 4 channel. I still think he is smoking crack. But, just in case, to get something in writing whether I am right or wrong, I wanted to ask you guys.

Thanks.

The one thing I would add is if you are going to use "tri-mode" you will need crossovers for all speakers, or at least your sub if you're okay with running your highs full range. 

Your subject title is worded wrong but I got what you are saying.

With the low cost of car audio amplifiers today, doing this would be nothing short of stupid.  But it is possible with a build-it-yourself passive crossover to separate the freqs between the four speakers and the sub.  Basically, it's a tri mode network.

Two speakers (coaxials or whatever) can be wired to one channel.  If each one is wired directly to the amplifier output, the wiring is parallel.  The selection of what speakers to use in this configuration is important, then, because you have to be sure to not overload the amplifier (as if it's not being overloaded enough already) with too low of an impedance.  Best bet would be to wire the two right side coaxials in series (if they are each 4 ohm, it would be an 8 ohm impedance).  If the speakers are 2 ohm impedance like some of the cheaper OEM replacements are now, they could be wired in series to make 4 ohms.  I would stay away from going as low as 2 ohms because of the subwoofer that will be bridged across both channels.  In a perfect world you probably could push 2 ohms/channel into these speakers, but when you are talking about milking every squeeze of available power out of one lone amplifier for a 4 speaker / one sub system, you are not likely using the best of equipment to begin with.  If you had the money for decent equipment you would just go out and buy the amplifiers you need.

So now there are four speakers connected to a two channel amp and they work left-to-right as you fiddle with the balance control (but you've lost the ability to fade front-to-back).  The sub can then be bridged across both channels (assuming this wunderkind amp is of the bridgeable variety), but you have to first build your passive crossover and solder the components into the speaker wires.  A first order crossover has a capacitor on the speaker lead and an inductor on the subwoofer lead.  See https://www.bcae1.com/trimode.htm for a diagram and better info than I can give you.

Nobody should ever be encouraged to use this type of configuration.  But as you can see, it IS possible.