My well is apx.400ft. with a reservoir created. The 3/4 hp. pump is submerged apx, 280 ft. Inside next to the 80 gal. water heater is located a blue water tank system thst is suppose to provides even more reserve water. My psi os set at 40. I'm not sure, but think we get 4.5 gal. per minute. My problem is that when excessive demand on the pressure en cures, the pump shuts dowm.
This occure if 2 toilets are flushed at the same time. I can not use a pressure washer, as it immediately shuts down. Is there any thing that can solve this problem?

I'm not an expert on well tanks but if the water pressure drops that fast then you probably have a bad well tank. They can loose their air bags sometimes and the water won't build up pressure.

How old is the tank?

There is likely a safety interlock on the pressure switch on the pump that shuts it down if the pressure drops too low. Mine is set at 20 psi. If the tank pressure drops below that it shuts the pump off. This is to prevent the pump from burning up if it cannot maintain minimum pressure. Mine is an old shallow well (53 feet), and if a commode fails to shut off, it sometimes pumps the well down to the point where the pump cannot build pressure. It sounds like you are probably not getting 4.5 gpm.Flushing two commodes should not tax the pump if it is delivering 4.5gpm. I would check take a 5 gallon bucket and fill it for 1 minute right at the pressure tank and see just what delivery you are getting. Then you can move on to remedy the problem. If the pump is old it may have some worn seals or an internal leak inside the pump. Usually a bad tank will show up if every time you open a faucet the pump kicks on because there is no air cushion in the tank.