I had a teacher who used to say "it's OK to cheat on tests, it's not OK to get caught." (gosh, sometimes I miss school)

So yeah, @Royalized, maybe you won't get caught, it doesn't mean that nas will be OK with that if/when he does. IP addresses are not the only way of detecting alts - in-game behavior can also be a good indication (e.g. giving away hundred of thousands of gil worth of items for nothing in exchange is a good indication of links between characters).
Also mind that such violation can potentially lead to both accounts being terminated and I'm not even sure whether nas himself has the means of restoring a character once deleted. From past experience (of others) violations are usually punished harshly (even though not the harshest) and without warning, e.g. there was someone just recently who violated a similar rule, by leveling his third character (which is allowed only as a mule). Once caught the character was immediately reduced back to level 1 and he didn't get to choose which character would suffer that fate (apparently the character that was deleted was not the least valuable character he had). Still I'd say that punishment could be even called considerate since his other two characters were left untouched. Judging by the same logic I'd guess (obviously it's just my guess) that if you get caught using two accounts then one of the accounts will be deleted without warning and you don't get to pick which one. You can go ahead and decide whether it's worth the risk.

Also, I have to say that I'm really interested why would you need two accounts in the first place? You already get two unrestricted characters (the alt character has no restrictions whatsoever, you can use it as your main if you want) and a mule. Unless you're trying to sell accounts (which I'm positive will get you deleted completely) how did you even reach a situation in which two and a half characters are not enough? I'm kinda lazy leveling one alt or even multiple subjobs - exactly how much time do you spend on FFXI?