Option to determine assigned hours manually in a task

Currently on a task when two users are assigned the hours determined for each User by system is predetermined. Meaning if a total of 10 hours are assigned for a task, each User will automatically be assigned 5 hours each. There isn't a way to determine for one User to be assigned 2 hours and the other User 8 hours; or 3 hours for a User and the the other User 7 hours.

We'd like a feature where assigning hours can be customized for each User on a task.