There is a salary cap in place but it is, technically, a soft cap. So saying that the Celtics can't have both Posey and Maggette is falacy. It simply depends on how much the owners are willing to pay luxury tax. If the owners are ok with paying an extra $5mil a year to match what they pay Magette or Posey then that's their business, and so far these owners have exhibited a willingness to spend money if the quality talent is there.
I think this was explained to you by multiple posters yesterday, but you're incorrect. While it is true that it's technically possible to sign both Maggette and Posey, we can't make them any contract offer we want.
Instead, any contract offer has to fall within certain limited "exceptions", which each have dollar caps put on them ($5.8m total for the MLE to be allocated as we see fit, the $1.91 million LLE, a "non-Bird" offer to Posey that is slightly less than $4 million next year, etc.)
You're implying that if the Celtics wanted, they could offer Posey and Maggette each, say, $6 million per season. That's simply inaccurate.
Again, I would implore you to check out
http://members.cox.net/lmcoon/salarycap.htm#19. People are already confused enough without the spread of misinformation, stated as fact.