Is it possible to dual class humans in the Dragonlance Goldbox games

No, it isn't. The Dragonlance setting uses a slightly modified ruleset when it comes to multiclassing (far more generous and flexible for non-human races) and dual-classing, which was omitted in these games; not only in the Dragonlance Goldbox games, but also the actual pen-and-paper rules on which they are based.