Well, multibox is pretty common in Lineage 2, so NCSoft and most players that migrated from there are used to the concept.

As far as "legit" way goes, most people just do the old window mode/alt-tab stuff. Or you could always go for the hardware method.

Gameguard is just too annoying to have it on, it causes all sort of unwanted crash and BSOD. But yeah, if you wanted to play it safe (TOS/EULA) better to just leave it on.