A family of Microsoft relational database management systems designed for ease of use.
Dharm,
I'm glad you gave the specifics because your opening sentence doesn't make sense. There is no, "mid way through an un-started task". The task doesn't start until somebody is working on it.
But to answer your question, yes, but you can't set it up so Project does it automatically, Project just doesn't have the level of "intelligence".
Here is your scenario as shown on the Gantt Chart view
And this is how it looks on the Task Usage view
After Res 1 completes effort on Task A, he/she is assigned full time to help Res 2 on Task B. The result is the duration of Task B is less because now two resources are working on it.
Hope this helps.
John