Can a SMPS transition between 2 unsynchronised AC power sources
You will need enough input capacitance, and It also depents on the load. You can try to calculate It but I Think it's easier to Just try it out. However I would not reccomend using handswitch, because it May be possible to vary switching time a lot. Instead Relay + latching circuit can be used.
Ac to dc smps works like this:

rectyficate Input 50Hz to dc (for 230V mains  it's 325V rectified)
switch this Dc with high frequency throught transformer (to transform it to needed voltage)
rectyficate high frequency voltage to dc at output.

All of this is done to reduce size and cost of such supply, because components working with higher frequency can be significantly smaller (eg. transformer).
To obtain smooth input dc voltage to swich it into transformer - capacitors are used. They allow converter to work continously even when input voltage pulses 100 times per second (mains). Their main task is to provide enough energy between halves of sine to allow for continous work of smps. BUT in most cases they have much bigger value (for example supply can work in multiplier input mode on 110V and then more capacitance is needed) This leads to the situation, when converter works for a moment, when it is unplugged from mains. You can use this fact to swich input source to smps, and it will not even notice a difference (voltage might start dropping on capacitors, but converter will then extend duty cycle to adjust to it, so if You are fast enough It's possible)