Decouple task list order from completion order?
 

I've got a bunch of tasks (call them group A) and each one is a prerequisite to one or more of a set of other tasks (group B). My task list looks like this:
[LIST=1][*]Group A[*]Group B[/LIST]
Group A are 3D modelling tasks, and B are programming tasks. They are assigned to 3D modellers and programmers, respectively.

After assigning resources and levelling, I get a plan that takes a very long time because OmniPlan insists on scheduling all of the tasks in A before any of the tasks in B. This results in a much longer schedule than if I manually interleave groups A and B.

Is there some way to get OmniPlan to optimize for project time rather than doing everything in the listed order? All tasks have the same priority right now.