Wiring two stereo channels to one sub?

I have a cheap Soundstorm F4.400 amp (4 channel, non bridgeable, 2 ohm capable).

I want to use it to drive two small drivers in stereo on channels 1 and 2, and to use channels 3 and 4 to power one small 4 ohm subwoofer. I realize that bridging is not going to work like on most higher end amplifiers. What if I combine the positive signal on the left and right and feed them to the sub, therefore giving each channel a 2 ohm load (which the amplifier is capable of doing apparently)? Would the amp survive that and give me mono, while keeping stereo on the other two channels?

Thanks for the replies, much appreciated. Nowhere in the manufacturer manual nor on the amp itself does it mention bridging capabilities though...

The amp has a lowpass filter switch for one pair of channels, and a highpass for the others pair, thus I wanted to run some small speakers with the high pass channels, and one sub with the lowpass channels.

How would one bridge it?

With the amp powered up (no audio input signal), what is the DC voltage on each of the speaker terminals (black probe on the amp's ground terminal)?

If they all have ~6v DC, the amp is a 'chip amp' and each channel is already bridged.

Driving the positive woofer terminal with the positive terminal of one channel and the negative woofer terminal with the negative terminal of the other channel would allow both channels to be reproduced by the woofer but would not increase the output (compared to connecting the woofer to either channel).
Well if its not bridgeable like mentioned here NEW SoundStorm F4.400 4-Channel 400W Car Amplifier/Amp - eBay (item 290514606588 end time Jan-19-11 20:48:32 PST) then you'll need a dual voicecoil sub like richie mentioned. If its not bridgeable yet is still 2 ohm stable a dual 2 ohm voice coil sub would draw the most power from the two sub channels. You would just wire one voice coil to each rear/sub channel.

Another way to check if the amp is bridgeable is to use a meter to check continuity between any two of the sub channels terminals (amp off). If two of the terminals a directly connected according to the meter (0.0-.9ohms approx), then the other two terminals would be the one you would bridge a single voice coil sub on.