One will "let out the magic smoke" per say.
Supplies can and are designed to operate in a master/slave arrangement, but the supplies have to be the identical model.
So, two 15 V, 50 A supplies could be connected in series and/or parallel, but the supplies have to be capable of doing so especially for the parallel situation.
In the series arrangement, one knob or whatever will adjust both supplies equally.
In the parallel arrangement, the setpoints again are the same, but one of the supplies operates in a manner that it's output becomes the setpoint to the other supply.
If you set the voltage to 11, the other supply will follow the 11V. If the master current limits, then the voltage of the master will decrease and, in tern, the setpoint to the slave will become the output of the master.
So, you can see how this could work.
If your batteries were as you had said, one of the battieries would charge to the higher voltage and might explode.