Choose your first channel you want to watch 215 then choose your other channel 242 using the number keys button, that should activate Doubleplay and can press arrow down by the Select button. using any other button key press would just turn off Doubleplay and clear the buffers.
BTW you have press down arrow twice when activating Doubleplay, which it will change to ch201 or ch 1, that's where you have to type in both the channel number you want then you hit down arrow button.