Depends on the PSU, if it cant handle it there is a chance that it dies and takes the 4870x2 with it, there is a chance that it just shuts down. You would need to jump their green wires together so that when one turns on the other one turns on as well.
What make and model are the two 450W units?
well i am not expert in this field but i will try ...
on psu lable it says make "INTEX"
DC output: +12V : 18A