How to combine multiple selects?

How can I combine these two rules into one?

#body_splash{
  width: 100%;
  height: 100%;
  display: none;
  opacity: 0;
}
#body_main{
  width: 100%;
  height: 100%;
  display: none;
  opacity: 0;
}

I tried

#body_splash #body_main

and

#body_splash#body_main

but neither worked