Connecting Two Stereo Amps together

Personally I wouldn't put two amps in series with each other.

You can only go as loud as the voltage rails of each amp, and even if your amps survive it, you'll be amplifing the distortion/noise from the first amplifier with the second one.

Unless someone else knows more, I don't think you'll gain anything, just noise.
In order to connect the amplifiers together, you could always use a bridge type configuration. The easiest way I believe is to build yourself two inverters and apply them to the inputs of both channels of one of your amplifiers, then you could bridge the configuration channel to channel.

However, when you really start to think about it...what can you really do with 150 watts that you can't do with 100? What is your application and/or need for a larger amplifier? Perhaps it isn't even necessary?

The only way to use 2 dissimilar amps on the same channel is in a biamp setup. Generally the lower powered one for the high frequencys and the higher power one for the low frequencys. This requires a crossover before the amps (this would be an active, low level crossover) and the speaker drivers are connected directly to each amp.