Thank you for the clarification, and I am sorry for putting the thread in the wrong section.
Don't be sorry about that, Hugues told you to post in the main section simply because only few of us regularly skim this section here. So, if you want to have a quicker and more detailed answer, it's better when your questions have a larger visibility :-)
As for your question, remember, you have to follow the rule of the phases: monsters are fought during the Movement phase. Each investigator starting from the first player resolves *entirely* his phase before the investigator on his left starts resolving the very same phase. This is why you cannot have multiple investigators fighting at the same time the same monster.
Hope this helps!