I note in Emergency Rules and the latest version of the SRD that “how many help dice are allowed on one roll” is left intentionally open to interpretation. But a couple 2400 installments (like Battle Moon) specifically say you only ever roll one skill die at a time, and if multiple apply, pick the highest or the one that fits best. Rolling an extra skill die could be the advantage of a special ability, though, like characteristics in Habs & Gardens, or a species characteristic (like “Aquatic: roll 2 skill dice when underwater”).