The most efficient method, is to create a second Windows user account through the Windows Control Panel.
This allows you to run two instances of either client, and UOAssist if running the Legacy client. If you are like me, I run legacy, and forego the Ehanced client because it makes me ill to play 3d games.
Simply log in on one account on one windows user account, then "Switch Users" on the windows taskbar. Rock and roll between the two instances. Not a hack nor a cheat. Not against the TOS. You are not using any third party proggie to cheat. You are using windows the way that it was designed to be used.