That comparison would be adapt because both laptops are portable (unlike a desktop) and so could be used on the go, where as a desktop is impractical to take with you everyday (even if Apple did sell a carrying case with the original Macintosh). So perhaps a better comparison is having a 13″ MacBook Pro (after all, the larger size does have more RAM and a USB 3 lightning port) and an 11″ MacBook Air. Well…not really.
The description fits until you consider how an iPad functions differently to an Laptop. Sure, if you are going to use your iPad purely as a writing device and with the smart keyboard at all times (never using the pencil and barely touching the screen) then maybe it would function as a laptop (and it does seem that Myke uses his larger pro more in this way though with a few significant differences). However, using an iPad is a very different experience. You can stand and walk around while reading in portrait (something harder on a larger device). You can relax in an armchair (as Steve Jobs showed us), you can draw over it’s screen, you can lay it out on your bed with your phone and switch devices as you look for different information, using it as a different piece of paper.
So I’m not sure the comparison between a dual desktop laptop setup really fits the iPad (until we get the 27″ all in one iPad that Apple said it will never make because it would tire your hands to use them in such a way) but perhaps there is truth that these different sizes lend themselves to different style of computing and tasks. The larger Pro helps you to use it more as a laptop, with larger full screen productivity apps, videos and games (but at a cost of portability), the small device has every so slightly less power but is vastly more portable and has less screen real estate.
Does that mean you should buy two iPads, well that’s your decision. If you work out a reason and have the cash then maybe.