When the key S is turned off, the circuit is the basic circuit of R2, and both ammeter A 1 and ammeter A2 measure the circuit current, indicating the same.

When the S key is closed, the resistor R 1 and R2 are connected in parallel, the S key controls the branch where R 1 is located, the voltmeter measures the power supply voltage, the ammeter A 1 measures the main current, and the ammeter A2 measures the branch current of R2.

(1) When R2 is disconnected, the voltmeter measures the power supply voltage, so the indication remains unchanged, and the indication of ammeter A2 is always zero, so the indication remains unchanged. The ammeter A 1 measures the current passing through the branch of R 1, so the indication becomes larger;

(2) When the resistor R 1 is turned off, the circuit is still the basic circuit of R2, so the indication of ammeter A2 remains unchanged, and the indication of ammeter A 1 remains unchanged. The voltmeter measures the supply voltage, so the voltage indicator remains unchanged.

When the resistors R 1 and R2 are open, the readings of ammeter A 1 and ammeter A2 are always zero, so the readings are unchanged. The voltmeter measures the power supply voltage, so the reading is unchanged.

So, the answers are: ①R2 is open; ②R 1 is open or both resistors R 1 and R2 are open.