I have one SSD on my computer, its 480GB.  Is it ok to put both sims on this SSD, is there a special set up to be done.  Should I buy a second SSD to run P3D, I am assuming I have enough space to put both on this, my current FSX is about 70GB with many add ons?

Yes - it's a lot safer than FSX and FSX :SE on the same drive

No problem with doing this, we have FSX, P3D and X-Plane 10 on a single SSD. It can get a bit confusing however with multiple versions of stuff like Ezdok, Orbx FTX Central etc. Just be mindful of how stuff is named and where it points to and you'll be fine.