Correct I only got the amp kit with the two boards and power supply, but no chassis. I was thinking that I could use Amp#1 to power the mains and Amp#2 to power the sub all from the same input source.
Would this simply be done by using a Y type cable to go to the inputs of the two different amps?
Sorry the questions are so dumb? 
No need to be sorry, that's how we all learn

Unfortunately, it's not quite that simple. Sorry for the bad drawing...
------>100hz and above (?)-------------------------------------->CDA 258 stereo----------->Main Speakers
/
/
preamp----->active crossover
\
\
----->99hz and below (?) left and right channels summed----->CDA 258 bridged mono-->Subwoofer
I picked 100hz, but whatever works best for your system. You don't want to let the speakers dictate the frequency roll off, and you need to be able to adjust the volume to your mains and subwoofer to get them to "blend".
Here's a link to one option, if you want a DIY approach. Also, much better illustrations!
http://www.marchandelec.com/ftp/xm9man.pdfMike