To be precise, both Melek and Kalamax do trigger simultaneously. However, they will resolve separately, which is why Staff makes two additional copies.Quote from NightWatch »would they each trigger Twinning Staff separately on the stack
See rules 603.3b & 608 for details.
Additionally, neither of them 'trigger' Twinning Staff. What actually happens is that Staff's ability will modify the effects of those two as they are happening.
See rule 614.1a for details.
Neither Melek nor Kalamax target anything. The word 'target' has very specific meaning, in Magic, and its misuse may lead to confusion.Quote from NightWatch »or would Twinning Staff only copy once since all of the copy triggers are targeting the same spell?