In serial mode, yes Channel 1 + is most positive, Channel 2 - is most negative and Ch 1 is bonded internally to Ch 2, but floating above mains earth. You can connect either Ch 1- or Ch 2 + to a green terminal to make that reference to mains earth.
In this mode, the Ch 2 controls set the voltage and current limits for both channels equally, Ch 1 controls have no effect. So to power, for example, an op amp, Ch1 + would be your positive rail, Ch1 - would be your ground plane and Ch2 - would be your negative rail. Both rails would be equal but opposite, controlled by Ch2 settings. Note that changing the settings in serial (or parallel) mode also changes the memory setting for Ch1 to the Ch2 settings.