normally two projects runs with my dualcore, but now i'm looking and Prime grid unit is on "waiting status" and continue to compute even if two other projects crunch their wus

it's not a big issue, but i'm wondering if this can be problematic...

Some of the PG WU's have a problem with being told to go to sleep, or to stop computing, and continue to run, even tho BOINC told them to pause. It doesn't happen with all of them, but I believe the 321 WU's do this from time to time.

It's not a BOINC problem, but a PrimeGrid problem. There are some PG forum entries about this.

I've seen Malaria units do this as well.
The only time this is a significant problem is if BOINC later re-loads the work unit with a checkpoint, and the WU runs twice. In this case, both keep overwriting the checkpoint file at different points, and so both keep resetting to the original because the checkpoint was "corrupt". If you kill one task, the other will finish.