TL:DR
READ YOUR PAPER WORK and Im sorry I am an engineer.. I like to hear myself talk
As it happens @MisteryAngel is correct.. Thank You @eidolonFIRE for the information on 2 channel bridging.
As it happens a bridgeable amp will clearly state that it can be done and how to do it.
For the sake of information in the post I will go into what bridging is
Bridging refers to combining two or four channels of an amplifier into one or two channels with twice the voltage. A two channel amp can be bridged to one channel, and a four channel amp into two channels if the manufacturer states that it can be bridged. Bridging the channels increases the overall real power output. An amplifier is usually bridged to combine two channels to power one subwoofer (This is the OP's case), or to combine four channels into powering two subwoofers. (My car's case) To clarify, you cannot bridge a monoblock amplifier. If you are purchasing a monoblock amplifier forget the idea as they only have one channel. In bridging you need opposing channels in order to increase the power output. In electrical engineering terms(my terms), you should be using a low source impedance to drive a large load impedance, which results in maximum voltage transfer. There are a couple pieces of mathematics here that could tell you how much voltage transfer you get but for the sake of today's discussion I do not think I am going to discuss the Thevenin's AC Max Power Theory.
Now if its bridgeable their will be some sort of directions on how to do so but you should theoretically be able to bridge the two channels and power the single channel subwoofer
Now im going to pick on boss audio right here... Basically all of their stuff in my opinion is pretty sub par and i feel they tweaked their numbers to make it sound cool. I have had a lot of friends get not even a year of use out of their product but hey its worth a try and a fun experiment so have fun with it. The head unit. Make sure you get a solid one so even if the free amp goes away you still have a good head unit that you can use with the next amp :)
Please Follow these tips laid out here:
1) before bridging an amplifier you dont know supports bridging... check for inverted channels. if it has an inverted identical channel... then it can be bridged
2) Only bridge an amplifer that can both handle and be stable at the increased power load.
3) ALWAYS ALWAYS ALWAYS study the diagramns and product paper work before attempting any such work
4) The General rule of thumb is that all well designed multi-channel amplifiers have a minimum bridged stability rating that is higher than the minimum stated impedance from one of its channels. EX: a 2 channel amplifier that is 3 ohm stable per channel would have a minimum impedance of 6 ohms when bridged. In reality, most amps are only stable at a 4 ohm load in mono/bridged configuration but thats my experience
Generally how its basically done:
you locate the amp terminals. For your 2-channel amplifier, you will see four terminals. A positive and a negative terminal for each channel. If you are bridging this to one subwoofer, you will connect one piece of speaker wire from the positive terminal of channel 1 to the speaker’s positive terminal. Next connect a speaker wire to the negative terminal of channel 2 to the speaker’s negative terminal. and voila boom bridged