Edit: Ok I just saw Murry's post. I truly wish I had someone with that depth of knowlege available as a mentor. Outstanding Murry!

This is all based on a novice's understanding of MSproject so there may be techniques I don't know yet.

The secret to MSproject is to understand that you don't assign durations to tasks. Project calculates them based on the hours allocated to the task and the % effort per person assigned to the task. (A task that requires 40 work hours to complet, with one person assigned at 50%, will spread over ten calendar days. Simple math right?)

The start times of tasks are dependent on both interdependencies of the tasks. (Task b must start after task a) and on the availability of resources. (People)

if Bob is assigned to task A at 100% and task B at 50%. Tasks A and B cannot overlap. No resource can be in 2 places at once. MSproject knows this and schedules the tasks so they don't overlap. Just make sure no one is over subscribed. As long as task A and B arn't dependent on each other, and don't require the same resource, they will overlap. (I'm not sure how ties are broken, by task numbers, I think.)

I think I answered what was asked, Yes No?

p.s. Project spreads resources across the eitire duration of the task. If you mean for someone to be devoted 50% for 2 cal. days of a longer task, you will need to break that task into smaller tasks and tie the tasks together.