This can be done using the Burst Mode on each channel. With the sine frequency of 27kHz, if you set the number of cycles in the Burst Mode to be 270,000 then the burst duration will be 10 seconds. You then set the Trigger Interval to 20 seconds, and you'll get the 10 seconds "on" and 10 seconds "off". Do the same for the second channel, but set the delay to 10 seconds. This will cause the 2nd channel to come on when the first channel turns off.
Unfortunately, there isn't a way of easily changing the amplitude on consecutive bursts. You can do this manually (adjust CH1 while CH2 is playing, and vice versa), or programmatically.