Airborne illness I wouldn't be too concerned about, but a fecal test isn't a bad idea either way. If they come into contact with surfaces that have been contaminated with feces and not sanitized it can pass infection. There are fungal infections that can spread through contact with contaminated surfaces as well. That said, many people have more than one dragon and they get time out of the tanks (not necessarily together). If you keep everything pretty well cleaned and sanitized, and wash your hands well between handling each dragon, then the risks are probably minimal.