My intuition says "no, of course not! what are you, a dumbass!?" Anyone with half a brain can realize that it leads to potential cheating. On one account, you could mainly download stuff, just barely maintaining the minimum ratio requirement. And on the other account, you can seed all of the torrents that you previously downloaded with the first account - thereby easily attaining power user status.
However, I cannot help but notice the problems that arise out of not allowing this. What about huge LANs, such as colleges and universities. To the WAN, each computer in the LAN has the same IP address. So would a private tracker only allow one account per LAN?
If this is to prevent users on smaller LANs from sharing their torrents with each other and in effect cheating the ratio system, couldn't you just as easily share torrents with your neighbor, or you friend from another neighborhood? So that isn't very effective.
On the other hand, there could be a way for private trackers to both allow accounts to share the same IP address by monitoring the accounts closely for things like seeding torrents that wasn't downloaded by that account.
Did I just answer my own question, or is there still more to this that I haven't realized?