It's most likely OK to do. I've done plenty of multiclienting with this game (one account apiece on two computers) in the same location (i.e. the same IP address on each client, which OGP could easily detect), and I haven't drawn any attention from OGP. There are also plenty of people who bring up multiple clients for running instances, abusing bard/minstrel buffs for grinding, self-leeching, etc.
As for two clients on the same computer, the game client has measures to prevent you from doing it.
If you want to be absolutely sure, try the OGP helpdesk. It shouldn't hurt to simply ask.