They would not stack
From the SRD:
Combining Magical Effects
The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don’t combine, however. Instead, the most potent effect—such as the highest bonus—from those castings applies while their durations overlap.
For example, if two clerics cast bless on the same target, that character gains the spell’s benefit only once; he or she doesn’t get to roll two bonus dice.
Although Ray of Frost's damage would apply with each hit, the speed reduction is an effect that the above rule would prevent from stacking with itself. However, it would still stack with other speed reducers like Lance of Lethargy or Frost Lance.