My mother in law is building a new house and when they poured the foundation they did not have enough concrete to finish so they are coming back in a few days. My concern is first, is this right or should they have pour all in one day and second what guide lines are there in pouring a slab over another slab. Please help.

It would be better as one pour, but if the reinforcement iron protrudes out of the set concrete and will be covered by the new. That will at least make a bond and maybe not weaken the foundation

My mother in law is building a new house and when they poured the foundation they did not have enough concrete to finish so they are coming back in a few days. My concern is first, is this right or should they have pour all in one day and second what guide lines are there in pouring a slab over another slab. Please help.
When they make the second pour, have then wet down the forms and existing concrete. THis will help the mud flow for a better bond and smoother form wall clean surface later. In small joining areas, oldtimers would use Elmer's glue for better bonding. (No Joke)