It can't really be "successful." Assuming the two successfully market the handhelds, and the games on each appeal and equal amount, they'd kind of both tie and come out even ("even" in the economic sense of the word, it might entail profit, but not more than they could've made elsewhere in the market). You're not losing money, but you're not making money either. Of course, this doesn't consider the fact that Nintendo has pretty much unmatched first party games when it comes to mass appeal and sales, so assuming the portables were relatively even, Nintendo would probably still win out.
In general, whenever a profitable company lacks competition, competitors will come and copy, and the market will even out for everyone (who isn't driven out of the market) until someone gains an advantage. This assumes equal products, marketing, etc. What makes that challenging to do in the game market is that it's very hard to get games that compete with Nintendo's first party stuff, and that's why no one can really "copy" Nintendo.
This is all Economy 101, and it's kind of pathetic to see people like Pachter completely miss it. The issue is not that Pachter doesn't understand business; it's that he doesn't understand games. If you can't understand games, you can't understand the biggest advantage a company can gain in this industry.