Is the Simultaneous Collapse of the wave functions (of 2 entangled particles) in violation of Relativity?

Not really. The collapse isn’t a physical phenomenon. Even the originator of the wave function (Schrodinger) urged us to treat it purely as a mathematical construct, devoid of physical reality.

But doesn’t the wave function correspond to some sort of physical reality for a single particle?

For a single particle, the configuration space (3 dimensional) is the same as it’s physical space. So the argument can be made that the wave function for a single particle, is, in some sense, real.

So, if it is real, doesn’t the simultaneous collapse of two wavefunctions violate the ‘No Special Frame of reference’ principle?

The wavefunction is simply a statistical description of the system. It’s collapse can be understood as a change of all available information about the system  – no more, no less.