Why would both toilets in the house be overflowing when flushed?

The drain pipe has a blockage somewhere downstream from where the toilets are connected to the main sewer line from the house. All the drains in a house normally connect to one main line that either goes to the city sewer system or your septic tank. If you know where the sewer line exits the house, you could check there to see if the blockage is in the yard. Most sewer blockages are caused by tree roots, broken fittings, or crushed pipes. Good Luck