If the amp is 1 ohm stable in mono mode, I say go for it. Sometimes the reliability factor will hurt you a little so give it a try, it can't hurt.
The sensitivity issue can get a little tricky because you are dealing with dB which are based on a logarithmic scale. If you double your amplifier power, you raise your SPL by 3dB, which is noticeable to some but not be everybody. In order to double your sound output, you need to 10x your amplifier power. 100w x 10=1000w to be twice as loud.
The other way you can add dBs to your vehicle is by adding cone area. Everytime you double your cone area you add three dB of sound.
So adding another sub will give you the same result as doubling the power to 1 sub, with a few advantages, lower distortion because the cone is not required to move as far, and lower power requirements. The disadvantages are needing twice the box size.
On a DVC sub, the measurements are usually taken with the VC wired in parallel. This will give you the highest sensitivity because you are lowering the resistance on the amp. On a 10w6 you would get the following...
VC in series 12ohms=not a lot of power from your amp
VCs in parallel=ideally 4 times the amplifier output.
So, you end up with a 6db increase in sensitivity because you are quadrupling the amplifier output.
Does this make sense?
