How can I split one activity
I have one activity that is a long term activity and I assign a resource to this activity, then I have a new critical activity that I need to switch the resource to this activity and then go back to this activity
Regards
Just create your new activity and set its planning priority to a lower value than the long term activity.
If you want to start new activity at a given future date, the best is to create a milestone as a "fixed" milestone to the given date, and define this milestone as predecessor of the activity.
Replan and you're in.
Regards.