You can put them in parallel but not series. When you put them in parallel, you may want to place a forward facing diode on the output side in order to not back feed into the PSU. They would very much not like to receive an overcharge, especially during heavy power loading.
In fact it may be better to push them both through an LDO or FET array system so that they can load balance on their own and independently. If two power supplies are in a system and they are not aware of each-other, they both could over step in adding more current on high demand and cause large voltage drops across the device. Any re-drive will most certainly be too much and kill the load.
