That would be kinda hard to tape two of the mini versions together. Because the PS2's DVD reader is the lid kind, just like the mini PS1. However, the PS1's DVD lid is smack dab in the center of the console, but the PS2 is to the right (or the left depending on how you want to look at it).
Combining the slimmer versions of the two systems would cause a compatibility issue between the two hardwares. Which would mean massive returns, angry consumers, and uber loss in profits... Which is not unusual for Sony apparently.