I can't find a manual online, so no definitive answer.
It depends on whether the two 12 V outputs are fully isolated from each other. If so, you can connect them in series. If not, you can't.
Check if there is a low resistive connection between any of the 2 outputs, i.e.:
GND1 <> GND2
Vout1 <> Vout2
GND1 <> Vout2
GND2 <> Vout1
You can use a run-of-the-mill multimeter in Ω setting for these measurements. Make sure the power to the power supply is off and any residual charge stored on internal capacitors is discharged (typically leaving the power supply unconnected over night will suffice). A resistance in the MΩ range would be o.k. A resistance in the kΩ range indicates the outputs are not fully isolated from each other.
Note that connecting the outputs in series will double the output voltage (you won't get "a bit more" unless the single outputs already show "a bit more" than 12 V) but will still limit the current to 7 A, the max. current of a single output.
