1. can we build two Huts in one "build" action?No, the base game rules say you pay 3 Vatus and place 1 Hut on the Island location.
2. can we build two Dikes in one "build" action?No, the rules say you may build "a dike" and phrase it similarly to building "a hut."
In general, the actions in Vanuatu can only be "done once" even if you have the resources to do them repeatedly, e.g. you can normally only buy & export 1 good. The notable exception in the base game is "sell fish," which explicitly states that you may sell "one or more fish tiles." I assume that if they meant to make it possible to build several huts and/or dikes, they would have said so.
(I also checked the unofficial rewording of the original expansions rules, and it too suggests you can only build one of each.)
Hope that helps