http://build.fhir.org/mapping-language.html#rules says:

> Dependent Rules: Optional part starting with 'then'. Specifies a group or set of child rules to apply within the scope of the rule.

By saying "a group", it's saying you can have just 1 group after then, whereas you can have multiple.

http://build.fhir.org/mapping-language.html#7.7.0.7.3 says:

> [...] Alternatively, a rule can nominate another group of rules from the same or an imported mapping. Each rule or group is listed by name, and then a set of parameters are provided.

This text does allow the possibility of multiple rules... but it's not super clear about it.

It'd be good to see the text clarified that you can indeed run multiple groups consequitively!