If tank2 was open to the atmosphere and there was an open line connecting tank1 to tank2, then the contents of tank1 would flow without limit into the tank2 leading to overflow.
If tank2 were sealed off from the atmosphere, then tank2 would be pressurized at the level of tank1.
If tank2 was sealed from the atmosphere, to allow tank2 to fill you would need a valve on top to bleed air until full and then you would close the valve.
You could have tank2 open to the atmosphere if you would have a "riser" tube that goes up above the top of tank1. In that case the pressure in tank1 would be the same as tank1.