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 difference between 150 W and 100 W is only 1.7 dB, which is only a marginally noticable difference. In any case, you cannot connect two different amps together in any way to get a single higher power output.
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.
HDTVman is "probably" right. You CAN put them in series, but only if they can take the increased current, which is unlikely unless you're using like a 16-ohm load. So biamping is your best bet. Or, of course, build a bigger amp