It goes without saying that a current source cannot be dumped directly into another current source, it needs a voltage sink/source.
Similarly, a voltage source cannot be dumped in another voltage source.
Since the PSUs are essentially working as a current source, a parallel combination seems reasonable but a direct series combination may result in voltage spikes at their connecting terminals during transients.
So terminating the 1st PSU wit a small capacitor and then connecting the other in series should help, provided of course that they are isolated internally.
The capacitor ensures that a current source is terminated in a voltage source and thus will absorb spikes during transients.