DC/DC isolated supplies in series for higher voltage?
As usual, the answer is yes and no.

At the theoretical text-book level the answer is yes.  At the practical level, there are some things that you'll need to check.

The #1 thing to check is what happens when one of the power supplies is on and the other is off.  This will for sure happen as you'll never be able to get both supplies to come up at the same time.  Let's say that your 48v supply comes up first.  That would essentially be "backfeeding" the +12v supply with -48 volts.  Some supplies might be OK with that, but I will guess that most will die a horrible death.  Without knowing the details of the supply, I would assume it would die.

Even if things were reversed, the +12v was on and the +48 was off, the +48v supply would be fed with -12v.  Even though +12 is smaller than +48, +12 is not the same as -12.  Again, without knowing more about the power supply I would assume you'd let the magic smoke out.

You could limit this somewhat by using schottky in parallel with each power supply.  This would limit the "backfeeding voltage" to Vf of the diode (about 0.5v).  Most supplies would survive this, but I wouldn't trust my life to it.