Running monoblocks in parallel to increase watts??

Is it possible to run monoblock amps in parallel so as to effectively increase their output wattage to each channel?? I have a set of monoblock amps now that are 100 watts per channel, and want to get a second pair of monoblocks so as to maybe have 200 watts per channel. They are tube monoblock amps, so will this effect impedence values etc??

I know this may take a special interconnect and speaker wire or at least the use of a splitter (or adding another set of output rca's on the preamp) to send the preamp signal to the pairs of amps (2 monoblocks for each channel = 4 monoblocks total) for each respective left and right channel. I assume that the output of the amps will have to "tie" together again (like biwire cable) in the run of speaker wire going from the four amps (2 per channel) to the speaker terminals?

If this possible, would I effectively be achieving 200 watts, or only like 150???

Thanks!

Robert