Setting priority of parallel tasks

I have a function, which to optimise it, I am going to have to split into two separate sequences of parallel processes (using parfeval). I want to ensure that the first sequence of parallel processes has no fewer than 2 processes and no more than 3 running at any time (until it runs out of processes to run of course), and for ALL spare workers to be running the second sequence of parallel processes.

Short of managing the parallel functions in a suitably intelligent loop, are there any tricks I can use to make this happen?