I got a original Samsung 2A output phone charger. If I open inside and add some wires and USB port to the circuit board and try to charge 2 devices in same time, what happens? Is it safe? Can I get two of 2A output in a same time?
That's not multiplexing.
That's just putting two devices in parallel.
It is common to power multiple devices from one powersupply.
It probably won't work well in this case, though.
You only have 2A, so if you try to connect two devices that each need 2A then your powersupply will only deliver 2A and the devices won't get the current they need.
Worst case, the charger burns out and causes a fire (unlikely if it really is from Samsung.)
Possibly it will shutdown if you put too much of a load on it. That would be safest.
Most likely it will deliver some current but not enough and then neither device would charge properly.
If the charger does any kind of communication with the devices, then you shouldn't try this at all. Some do, some don't.
Do not open the case. From your question, you would seem to lack the experience to work on line voltage safely.
You can put two cables in parallel without opening the case. If you want to try powering your two devices from one supply, do it that way. You will only have to deal with the (relatively) safe 5V on the USB cable.