Just because it's not in print doesn't mean it's deliberately omitted.
The amount of heavy lifting is simple:
"Weapons declaration for all units are written down to indicate the target for each weapon fired, the weapon's firing mode (IE double or single fire for Ultra ACs) and/or special ammo used. It is recommended to also notate the to-hit number calculated during the declaration, to speed up resolution subsequently. Weapons fire is then resolved using the normal rules."
All weapons fire is already simultaneous, so no additional rules are needed.