Two power supplies connected together
It should be "ok" in steady state.

Theoretically when they are tied together like that, whichever one has the higher reference point will drive the circuit, the other will "throttle back", as it were, since it will think the output is too high.

However, if the reference points are really close or overlap in their hysteresis, there is the real possibility that you could set up an oscillation between them. On the load side that would appear as some form of ripple and the chips might get a bit warmer than usual.  As such you may want for force one to be a little higher than the other by offsetting the reference voltage through a shotky diode or resistor divider. 

The other got-ya in there is the inductor in the 12V side, if the battery side has the higher reference. And to a lesser extent, vica-versa.

If the 12V is connected first then there will be significant current flowing in the inductor. As such when you plug in the battery and that circuit ramps up, that current may be in excess of the load requirement. 

There may be a short voltage spike generated at that time. You need sufficient bulk capacitance to absorb that. It may also be prudent to add an over-voltage limiting Zener across the output just to be sure.

BTW: it is common practice to use a switched connector that disconnects the on-board battery circuit when plugging in a wired power source.