You don't need a special client or a seperate laptop lol, just open two pages with the names in the lobby with the world you want them logged into. Login on the names 'any amount' all at the same time on that world, bam, easy.
You CAN get caught and banned as you are not allowed to transfer money to different accounts. But it is extremely unlikely unless you are doing mass amounts multiple times.
Multi Logging is Illegal but, you can't get in trouble for it. RS just likes to troll and scare people so they don't use the Multi one-second logg cheat.