Whether this can be done *could* depend on the power supplies. While it will work fine with batteries (since there's no ground "reference"), I've seen cases where power supplies can't be "stacked", since the negative output is connected to earth ground. If this is the case, connecting them in this way will short-out the first supply. Likewise, if the negative terminal is connected to the PS case, another set of problems & safety issues would be presented when stacking the PS's.
So, before connecting this way, I'd suggest verifying that the negative terminals are isolated from the PS cases and, separately, earth ground.
--Dave