I didn't find my system on the list (HT-DDW900 (STR-K900,
RM-AAP013)), but based on the information posted for other
systems, I poked around and found a procedure that worked
for me.

On the receiver (STR-K900):

1. Press and hold the INPUT MODE button.
2. While holding the INPUT MODE button, press the Power
(I/O) button.
3. Each time you do this, the command mode will toggle
between AV1 and AV2.

On the remote (RM-AAP013):
1. Press VIDEO1 button (for AV1) or the VIDEO2 button
(for AV2) while pressing the RM SET UP button
2. The RM SET UP button flashes once for AV1 or twice
for AV2, then the command mode switches.