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Brian beat me to the punch, although I might have started my answer before he did, and mine has pictures :-)

I'll add to what he gave you. You've already got the answer for how to control whether duration or effort changes when you alter the resource mix, but that doesn't quite cover your whole problem. You need to tell OmniPlan that you only want to give 20% of those resources to those tasks because the other 80% of the time you want to devote to other tasks (in parallel with the long one). You can edit this in the inspector as shown in the screen capture here, and you can also edit it in the Gantt chart. There's probably yet another way that I'm forgetting.

The lengthy task is assigned at 20% for both Curly and Moe, so they are able to use the other 80% to work on Tasks 1 and 2 initially, filling their schedule to capacity, as seen below:

Tasks 3 and 4 I've assigned only 50% effort, so you can see that a task requiring the same effort is taking additional duration, and looking at the utilization graph in the second image, they aren't fully utilized during that time. Note that you have to specify the lower percentage in assignments or you may get a violation (trying to assign Curly at 100% of 100% for Task 3 prior to the completion of the lengthy task, for example) or the leveling code will schedule it for when there are no other demands on the resource. It would be nice if OmniPlan could simply say "oh, he obviously wants this resource to spend all its remaining effort on this task" and do the adjustment for you, but it doesn't.

It strikes me as possible that you didn't really have a fixed duration for this task so much as you'd decided that it was about 32 hours of work, which is 4 weeks of work at 20% effort, knowing that you had 80% of effort of other stuff you wanted Curly and Moe to be working on. It is tempting (to me, at least) to think of the resource allocation as filling a channel through the project, but you have to remember that it uses the smallest channel width that fits all the way, and doesn't make any bulges when the "walls" get farther apart. Assuming my hypothesis about the lengthy task is correct, in real life once Tasks 1-4 in the diagram above finished, Curly and Moe would start working all day on the lengthy task if they didn't have other things to do, but OmniPlan will never give you such a plan unless you split the lengthy task.